[RFC FS-1060] Nullness checking (applied to codebase) (#15310)

- apply nullness to codebase
- build F.C.S also againts net9 to get best annotations (multi targetting)
- bootstrap FSharp.Build in proto build

.fantomasignore

@@ -12,7 +12,7 @@ vsintegration/*
 !vsintegration/tests/FSharp.Editor.Tests
 artifacts/
 
-# For some reason, it tries to format files from remotes (Processing .\.git\refs\remotes\<remote>\FSComp.fsi)
+# For some reason, it tries to format files from remotes (Processing ./.git/refs/remotes/<remote>/FSComp.fsi)
 .git/
 
 # Explicitly unformatted implementation
@@ -101,10 +101,22 @@ src/FSharp.Core/option.fsi
 src/FSharp.Core/option.fs
 src/fsi/console.fs
 src/FSharp.Build/FSharpCommandLineBuilder.fs
+
+src/Compiler/Utilities/Activity.fs
 src/Compiler/Utilities/sformat.fs
 src/Compiler/Utilities/illib.fsi
 src/Compiler/Utilities/illib.fs
 
+
+src/Compiler/Utilities/NullnessShims.fs
+src/Compiler/Utilities/LruCache.fsi
+src/Compiler/Utilities/LruCache.fs
+src/Compiler/Utilities/HashMultiMap.fsi
+src/Compiler/Utilities/HashMultiMap.fs
+src/Compiler/Facilities/AsyncMemoize.fsi
+src/Compiler/Facilities/AsyncMemoize.fs
+src/Compiler/AbstractIL/il.fs
+
 # Fantomas limitations on implementation files  (to investigate)
 
 src/Compiler/AbstractIL/ilwrite.fs

FSharpBuild.Directory.Build.targets

@@ -88,6 +88,14 @@
     </ItemGroup>
   </Target>
 
+      <!-- SDK targets override -->
+  <PropertyGroup Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')">
+    <FSharpBuildAssemblyFileOverride>$(ProtoOutputPath)\fsc\FSharp.Build.dll</FSharpBuildAssemblyFileOverride>
+  </PropertyGroup>  
+  <UsingTask TaskName="FSharpEmbedResourceText" AssemblyFile="$(FSharpBuildAssemblyFileOverride)" Override="true" Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')" />
+  <UsingTask TaskName="FSharpEmbedResXSource" AssemblyFile="$(FSharpBuildAssemblyFileOverride)" Override="true" Condition="'$(Configuration)' != 'Proto' AND '$(DisableCompilerRedirection)'!='true' AND Exists('$(ProtoOutputPath)')" />   
+
+
   <Target Name="BeforeResGen"
           Inputs="@(EmbeddedResource->'$(IntermediateOutputPath)%(Filename)%(Extension)')"
           Outputs="@(EmbeddedResource->'$(IntermediateOutputPath)resources\%(Filename)%(Extension)')"

azure-pipelines-PR.yml

@@ -553,6 +553,8 @@ stages:
           - checkout: self
             clean: true
           - script: ./eng/cibuild.sh --configuration $(_BuildConfig) --testcoreclr
+            env:
+              SKIP_NETCURRENT_FSC_BUILD: true
             displayName: Build / Test
           - task: PublishTestResults@2
             displayName: Publish Test Results
@@ -595,6 +597,7 @@ stages:
           - script: ./eng/cibuild.sh --configuration $(_BuildConfig) --testcoreclr
             env:
               COMPlus_DefaultStackSize: 1000000
+              SKIP_NETCURRENT_FSC_BUILD: true
             displayName: Build / Test
           - task: PublishTestResults@2
             displayName: Publish Test Results

docs/release-notes/.FSharp.Compiler.Service/9.0.100.md

@@ -25,6 +25,7 @@
 * Treat `{ new Foo() }` as `SynExpr.ObjExpr` ([PR #17388](https://github.com/dotnet/fsharp/pull/17388))
 * Optimize metadata reading for type members and custom attributes. ([PR #17364](https://github.com/dotnet/fsharp/pull/17364))
 * Enforce `AttributeTargets` on unions. ([PR #17389](https://github.com/dotnet/fsharp/pull/17389))
+* Applied nullable reference types to FSharp.Compiler.Service itself ([PR #15310](https://github.com/dotnet/fsharp/pull/15310))
 * Ensure that isinteractive multi-emit backing fields are not public. ([Issue #17439](https://github.com/dotnet/fsharp/issues/17438)), ([PR #17439](https://github.com/dotnet/fsharp/pull/17439))
 * Enable FSharp 9.0 Language Version ([Issue #17497](https://github.com/dotnet/fsharp/issues/17438)), [PR](https://github.com/dotnet/fsharp/pull/17500)))
 * Better error reporting for unions with duplicated fields. ([PR #17521](https://github.com/dotnet/fsharp/pull/17521))

src/Compiler/AbstractIL/il.fs

@@ -170,7 +170,11 @@ let splitTypeNameRight nm =
 // --------------------------------------------------------------------
 
 /// This is used to store event, property and field maps.
-type LazyOrderedMultiMap<'Key, 'Data when 'Key: equality>(keyf: 'Data -> 'Key, lazyItems: InterruptibleLazy<'Data list>) =
+type LazyOrderedMultiMap<'Key, 'Data when 'Key: equality
+#if !NO_CHECKNULLS
+    and 'Key:not null
+#endif
+    >(keyf: 'Data -> 'Key, lazyItems: InterruptibleLazy<'Data list>) =
 
     let quickMap =
         lazyItems
@@ -515,7 +519,8 @@ type ILAssemblyRef(data) =
 
         let retargetable = aname.Flags = AssemblyNameFlags.Retargetable
 
-        ILAssemblyRef.Create(aname.Name, None, publicKey, retargetable, version, locale)
+        let name = match aname.Name with | null -> aname.FullName | name -> name
+        ILAssemblyRef.Create(name, None, publicKey, retargetable, version, locale)
 
     member aref.QualifiedName =
         let b = StringBuilder(100)
@@ -823,7 +828,7 @@ type ILTypeRef =
     member x.DebugText = x.ToString()
 
     /// For debugging
-    override x.ToString() = x.FullName
+    override x.ToString() : string = x.FullName
 
 and [<StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugText}")>] ILTypeSpec =
     {
@@ -875,7 +880,7 @@ and [<StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugT
         && (x.GenericArgs = y.GenericArgs)
 
     override x.ToString() =
-        x.TypeRef.ToString() + if isNil x.GenericArgs then "" else "<...>"
+        x.TypeRef.FullName + if isNil x.GenericArgs then "" else "<...>"
 
 and [<RequireQualifiedAccess; StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugText}")>] ILType =
     | Void
@@ -1017,8 +1022,9 @@ type ILMethodRef =
     [<DebuggerBrowsable(DebuggerBrowsableState.Never)>]
     member x.DebugText = x.ToString()
 
-    override x.ToString() =
-        x.DeclaringTypeRef.ToString() + "::" + x.Name + "(...)"
+    member x.FullName = x.DeclaringTypeRef.FullName + "::" + x.Name + "(...)"
+
+    override x.ToString() = x.FullName
 
 [<StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugText}")>]
 type ILFieldRef =
@@ -1033,7 +1039,7 @@ type ILFieldRef =
     member x.DebugText = x.ToString()
 
     override x.ToString() =
-        x.DeclaringTypeRef.ToString() + "::" + x.Name
+        x.DeclaringTypeRef.FullName + "::" + x.Name
 
 [<StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugText}")>]
 type ILMethodSpec =
@@ -1072,7 +1078,7 @@ type ILMethodSpec =
     [<DebuggerBrowsable(DebuggerBrowsableState.Never)>]
     member x.DebugText = x.ToString()
 
-    override x.ToString() = x.MethodRef.ToString() + "(...)"
+    override x.ToString() = x.MethodRef.FullName + "(...)"
 
 [<StructuralEquality; StructuralComparison; StructuredFormatDisplay("{DebugText}")>]
 type ILFieldSpec =
@@ -1213,7 +1219,7 @@ type ILAttribute =
     [<DebuggerBrowsable(DebuggerBrowsableState.Never)>]
     member x.DebugText = x.ToString()
 
-    override x.ToString() = x.Method.ToString() + "(...)"
+    override x.ToString() = x.Method.MethodRef.FullName
 
 [<NoEquality; NoComparison; Struct>]
 type ILAttributes(array: ILAttribute[]) =
@@ -1571,7 +1577,7 @@ type ILFieldInit =
         | ILFieldInit.UInt64 u64 -> box u64
         | ILFieldInit.Single ieee32 -> box ieee32
         | ILFieldInit.Double ieee64 -> box ieee64
-        | ILFieldInit.Null -> (null :> Object)
+        | ILFieldInit.Null -> (null :> objnull)
 
 // --------------------------------------------------------------------
 // Native Types, for marshalling to the native C interface.

src/Compiler/AbstractIL/il.fsi

@@ -237,6 +237,8 @@ type ILTypeRef =
 
     member internal EqualsWithPrimaryScopeRef: ILScopeRef * obj -> bool
 
+    override ToString: unit -> string
+
     interface System.IComparable
 
 /// Type specs and types.
@@ -664,7 +666,7 @@ type ILFieldInit =
     | Double of double
     | Null
 
-    member AsObject: unit -> obj
+    member AsObject: unit -> objnull
 
 [<RequireQualifiedAccess; StructuralEquality; StructuralComparison>]
 type internal ILNativeVariant =

src/Compiler/AbstractIL/ilnativeres.fs

@@ -1000,7 +1000,7 @@ type Directory(name, id) =
     member val ID = id
     member val NumberOfNamedEntries = Unchecked.defaultof<uint16> with get, set
     member val NumberOfIdEntries = Unchecked.defaultof<uint16> with get, set
-    member val Entries = List<obj>()
+    member val Entries = List<objnull>()
 
 type NativeResourceWriter() =
     static member private CompareResources (left: Win32Resource) (right: Win32Resource) =
@@ -1149,7 +1149,12 @@ type NativeResourceWriter() =
                             dataWriter.WriteByte 0uy
 
                         false
-                    | e -> failwithf "Unknown entry %s" (if isNull e then "<NULL>" else e.GetType().FullName)
+                    | e ->
+                        failwithf
+                            "Unknown entry %s"
+                            (match e with
+                             | null -> "<NULL>"
+                             | e -> e.GetType().FullName)
 
                 if id >= 0 then
                     writer.WriteInt32 id

src/Compiler/AbstractIL/ilpars.fsy

@@ -3,6 +3,7 @@
 %{
 
 #nowarn "1182"  // the generated code often has unused variable "parseState"
+#nowarn "3261"  // the generated code would need to properly annotate nulls, e.g. changing System.Object to `obj|null`
 
 open Internal.Utilities.Library
 

src/Compiler/AbstractIL/ilread.fs

@@ -941,11 +941,11 @@ let mkCacheGeneric lowMem _inbase _nm (sz: int) =
         fun f (idx: 'T) ->
             let cache =
                 match cache with
-                | Null ->
+                | null ->
                     let v = ConcurrentDictionary<_, _>(Environment.ProcessorCount, sz)
                     cache <- v
                     v
-                | NonNull v -> v
+                | v -> v
 
             match cache.TryGetValue idx with
             | true, v ->

src/Compiler/AbstractIL/ilreflect.fs

@@ -163,7 +163,12 @@ type TypeBuilder with
         if logRefEmitCalls then
             printfn "typeBuilder%d.CreateType()" (abs <| hash typB)
 
+        //Buggy annotation in ns20, will not be fixed.
+#if NETSTANDARD && !NO_CHECKNULLS
+        !!(typB.CreateTypeInfo()) :> Type
+#else
         typB.CreateTypeInfo() :> Type
+#endif
 
     member typB.DefineNestedTypeAndLog(name, attrs) =
         let res = typB.DefineNestedType(name, attrs)
@@ -270,10 +275,9 @@ type TypeBuilder with
             else
                 null
 
-        if not (isNull m) then
-            m.Invoke(null, args)
-        else
-            raise (MissingMethodException nm)
+        match m with
+        | null -> raise (MissingMethodException nm)
+        | m -> m.Invoke(null, args)
 
     member typB.SetCustomAttributeAndLog(cinfo, bytes) =
         if logRefEmitCalls then
@@ -284,9 +288,12 @@ type TypeBuilder with
 type OpCode with
 
     member opcode.RefEmitName =
-        (string (Char.ToUpper(opcode.Name[0])) + opcode.Name[1..])
-            .Replace(".", "_")
-            .Replace("_i4", "_I4")
+        match opcode.Name with
+        | null -> ""
+        | name ->
+            (string (Char.ToUpper(name[0])) + name[1..])
+                .Replace(".", "_")
+                .Replace("_i4", "_I4")
 
 type ILGenerator with
 
@@ -320,7 +327,7 @@ type ILGenerator with
 
         ilG.BeginFinallyBlock()
 
-    member ilG.BeginCatchBlockAndLog ty =
+    member ilG.BeginCatchBlockAndLog(ty: Type) =
         if logRefEmitCalls then
             printfn "ilg%d.BeginCatchBlock(%A)" (abs <| hash ilG) ty
 
@@ -396,7 +403,7 @@ type ILGenerator with
 
     member x.EmitAndLog(op: OpCode, v: ConstructorInfo) =
         if logRefEmitCalls then
-            printfn "ilg%d.Emit(OpCodes.%s, constructor_%s)" (abs <| hash x) op.RefEmitName v.DeclaringType.Name
+            printfn "ilg%d.Emit(OpCodes.%s, constructor_%s)" (abs <| hash x) op.RefEmitName (!!v.DeclaringType).Name
 
         x.Emit(op, v)
 
@@ -693,7 +700,7 @@ let rec convTypeSpec cenv emEnv preferCreated (tspec: ILTypeSpec) =
     let typT = convTypeRef cenv emEnv preferCreated tspec.TypeRef
     let tyargs = List.map (convTypeAux cenv emEnv preferCreated) tspec.GenericArgs
 
-    let res =
+    let res: Type MaybeNull =
         match isNil tyargs, typT.IsGenericType with
         | _, true -> typT.MakeGenericType(List.toArray tyargs)
         | true, false -> typT
@@ -706,7 +713,7 @@ let rec convTypeSpec cenv emEnv preferCreated (tspec: ILTypeSpec) =
 
 and convTypeAux cenv emEnv preferCreated ty =
     match ty with
-    | ILType.Void -> Type.GetType("System.Void")
+    | ILType.Void -> !! Type.GetType("System.Void")
     | ILType.Array(shape, eltType) ->
         let baseT = convTypeAux cenv emEnv preferCreated eltType
         let nDims = shape.Rank
@@ -844,26 +851,10 @@ let queryableTypeGetField _emEnv (parentT: Type) (fref: ILFieldRef) =
     | NonNull res -> res
 
 let nonQueryableTypeGetField (parentTI: Type) (fieldInfo: FieldInfo) : FieldInfo =
-    let res =
-        if parentTI.IsGenericType then
-            TypeBuilder.GetField(parentTI, fieldInfo)
-        else
-            fieldInfo
-
-    match res with
-    | Null ->
-        error (
-            Error(
-                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen (
-                    "field",
-                    fieldInfo.Name,
-                    parentTI.AssemblyQualifiedName,
-                    parentTI.Assembly.FullName
-                ),
-                range0
-            )
-        )
-    | NonNull res -> res
+    if parentTI.IsGenericType then
+        TypeBuilder.GetField(parentTI, fieldInfo)
+    else
+        fieldInfo
 
 let convFieldSpec cenv emEnv fspec =
     let fref = fspec.FieldRef
@@ -1012,21 +1003,16 @@ let queryableTypeGetMethod cenv emEnv parentT (mref: ILMethodRef) : MethodInfo =
 
         let methInfo =
             try
-                parentT.GetMethod(
-                    mref.Name,
-                    cconv ||| BindingFlags.Public ||| BindingFlags.NonPublic,
-                    null,
-                    argTs,
-                    (null: ParameterModifier[] MaybeNull)
-                )
+                parentT.GetMethod(mref.Name, cconv ||| BindingFlags.Public ||| BindingFlags.NonPublic, null, argTs, null)
             // This can fail if there is an ambiguity w.r.t. return type
             with _ ->
                 null
 
-        if (isNotNull methInfo && equalTypes resT methInfo.ReturnType) then
-            methInfo
-        else
-            queryableTypeGetMethodBySearch cenv emEnv parentT mref
+        match methInfo with
+        | null -> queryableTypeGetMethodBySearch cenv emEnv parentT mref
+        | m when equalTypes resT m.ReturnType -> m
+        | _ -> queryableTypeGetMethodBySearch cenv emEnv parentT mref
+
     else
         queryableTypeGetMethodBySearch cenv emEnv parentT mref
 
@@ -1062,7 +1048,12 @@ let convMethodRef cenv emEnv (parentTI: Type) (mref: ILMethodRef) =
     | Null ->
         error (
             Error(
-                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen ("method", mref.Name, parentTI.FullName, parentTI.Assembly.FullName),
+                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen (
+                    "method",
+                    mref.Name,
+                    parentTI.FullName |> string,
+                    parentTI.Assembly.FullName |> string
+                ),
                 range0
             )
         )
@@ -1103,7 +1094,12 @@ let queryableTypeGetConstructor cenv emEnv (parentT: Type) (mref: ILMethodRef) =
     | Null ->
         error (
             Error(
-                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen ("constructor", mref.Name, parentT.FullName, parentT.Assembly.FullName),
+                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen (
+                    "constructor",
+                    mref.Name,
+                    parentT.FullName |> string,
+                    parentT.Assembly.FullName |> string
+                ),
                 range0
             )
         )
@@ -1138,7 +1134,12 @@ let convConstructorSpec cenv emEnv (mspec: ILMethodSpec) =
     | Null ->
         error (
             Error(
-                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen ("constructor", "", parentTI.FullName, parentTI.Assembly.FullName),
+                FSComp.SR.itemNotFoundInTypeDuringDynamicCodeGen (
+                    "constructor",
+                    "",
+                    parentTI.FullName |> string,
+                    parentTI.Assembly.FullName |> string
+                ),
                 range0
             )
         )
@@ -1490,7 +1491,7 @@ let rec emitInstr cenv (modB: ModuleBuilder) emEnv (ilG: ILGenerator) instr =
             ilG.EmitAndLog(OpCodes.Ldelema, convType cenv emEnv ty)
         else
             let arrayTy = convType cenv emEnv (ILType.Array(shape, ty))
-            let elemTy = arrayTy.GetElementType()
+            let elemTy = !! arrayTy.GetElementType()
             let argTys = Array.create shape.Rank typeof<int>
             let retTy = elemTy.MakeByRefType()
 
@@ -1516,7 +1517,7 @@ let rec emitInstr cenv (modB: ModuleBuilder) emEnv (ilG: ILGenerator) instr =
             ilG.EmitAndLog(OpCodes.Stelem, convType cenv emEnv ty)
         else
             let arrayTy = convType cenv emEnv (ILType.Array(shape, ty))
-            let elemTy = arrayTy.GetElementType()
+            let elemTy = !! arrayTy.GetElementType()
 
             let meth =
                 modB.GetArrayMethodAndLog(
@@ -1624,7 +1625,7 @@ let emitCode cenv modB emEnv (ilG: ILGenerator) (code: ILCode) =
 
         | ILExceptionClause.FilterCatch((startFilter, _), (startHandler, endHandler)) ->
             add startFilter ilG.BeginExceptFilterBlockAndLog
-            add startHandler (fun () -> ilG.BeginCatchBlockAndLog null)
+            add startHandler (fun () -> ilG.BeginCatchBlockAndLog Unchecked.defaultof<_>)
             add endHandler ilG.EndExceptionBlockAndLog
 
         | ILExceptionClause.TypeCatch(ty, (startHandler, endHandler)) ->
@@ -1830,24 +1831,25 @@ let rec buildMethodPass2 cenv tref (typB: TypeBuilder) emEnv (mdef: ILMethodDef)
         let methB =
             System.Diagnostics.Debug.Assert(not (isNull definePInvokeMethod), "Runtime does not have DefinePInvokeMethod") // Absolutely can't happen
 
-            definePInvokeMethod.Invoke(
-                typB,
-                [|
-                    mdef.Name
-                    p.Where.Name
-                    p.Name
-                    attrs
-                    cconv
-                    retTy
-                    null
-                    null
-                    argTys
-                    null
-                    null
-                    pcc
-                    pcs
-                |]
-            )
+            (!!definePInvokeMethod)
+                .Invoke(
+                    typB,
+                    [|
+                        mdef.Name
+                        p.Where.Name
+                        p.Name
+                        attrs
+                        cconv
+                        retTy
+                        null
+                        null
+                        argTys
+                        null
+                        null
+                        pcc
+                        pcs
+                    |]
+                )
             :?> MethodBuilder
 
         methB.SetImplementationFlagsAndLog implflags
@@ -2473,7 +2475,7 @@ let defineDynamicAssemblyAndLog (asmName, flags, asmDir: string) =
 
     asmB
 
-let mkDynamicAssemblyAndModule (assemblyName, optimize, collectible) =
+let mkDynamicAssemblyAndModule (assemblyName: string, optimize, collectible) =
     let asmDir = "."
     let asmName = AssemblyName()
     asmName.Name <- assemblyName
@@ -2490,7 +2492,7 @@ let mkDynamicAssemblyAndModule (assemblyName, optimize, collectible) =
         let daType = typeof<System.Diagnostics.DebuggableAttribute>
 
         let daCtor =
-            daType.GetConstructor [| typeof<System.Diagnostics.DebuggableAttribute.DebuggingModes> |]
+            !! daType.GetConstructor([| typeof<System.Diagnostics.DebuggableAttribute.DebuggingModes> |])
 
         let daBuilder =
             CustomAttributeBuilder(

src/Compiler/AbstractIL/ilsign.fs

@@ -64,7 +64,9 @@ let hashAssembly (peReader: PEReader) (hashAlgorithm: IncrementalHash) =
     let checkSumOffset = peHeaderOffset + 0x40 // offsetof(IMAGE_OPTIONAL_HEADER, CheckSum)
 
     let securityDirectoryEntryOffset, peHeaderSize =
-        match peHeaders.PEHeader.Magic with
+        let header = peHeaders.PEHeader |> nullArgCheck (nameof peHeaders.PEHeader)
+
+        match header.Magic with
         | PEMagic.PE32 -> peHeaderOffset + 0x80, 0xE0 // offsetof(IMAGE_OPTIONAL_HEADER32, DataDirectory[IMAGE_DIRECTORY_ENTRY_SECURITY]), sizeof(IMAGE_OPTIONAL_HEADER32)
         | PEMagic.PE32Plus -> peHeaderOffset + 0x90, 0xF0 // offsetof(IMAGE_OPTIONAL_HEADER64, DataDirectory[IMAGE_DIRECTORY_ENTRY_SECURITY]), sizeof(IMAGE_OPTIONAL_HEADER64)
         | _ -> raise (BadImageFormatException(getResourceString (FSComp.SR.ilSignInvalidMagicValue ())))
@@ -87,7 +89,9 @@ let hashAssembly (peReader: PEReader) (hashAlgorithm: IncrementalHash) =
     hashAlgorithm.AppendData(allHeaders, 0, allHeadersSize)
 
     // Hash content of all sections
-    let signatureDirectory = peHeaders.CorHeader.StrongNameSignatureDirectory
+    let signatureDirectory =
+        let corHeader = peHeaders.CorHeader |> nullArgCheck (nameof peHeaders.CorHeader)
+        corHeader.StrongNameSignatureDirectory
 
     let signatureStart =
         match peHeaders.TryGetDirectoryOffset signatureDirectory with
@@ -186,10 +190,10 @@ let toCLRKeyBlob (rsaParameters: RSAParameters) (algId: int) : byte array =
     if isNull rsaParameters.Modulus then
         raise (CryptographicException(String.Format(getResourceString (FSComp.SR.ilSignInvalidRSAParams ()), "Modulus")))
 
-    if isNull rsaParameters.Exponent || rsaParameters.Exponent.Length > 4 then
+    if isNull rsaParameters.Exponent || (!!rsaParameters.Exponent).Length > 4 then
         raise (CryptographicException(String.Format(getResourceString (FSComp.SR.ilSignInvalidRSAParams ()), "Exponent")))
 
-    let modulusLength = rsaParameters.Modulus.Length
+    let modulusLength = (!!rsaParameters.Modulus).Length
     let halfModulusLength = (modulusLength + 1) / 2
 
     // We assume that if P != null, then so are Q, DP, DQ, InverseQ and D and indicate KeyPair RSA Parameters
@@ -227,29 +231,37 @@ let toCLRKeyBlob (rsaParameters: RSAParameters) (algId: int) : byte array =
         let expAsDword =
             let mutable buffer = int 0
 
-            for i in 0 .. rsaParameters.Exponent.Length - 1 do
-                buffer <- (buffer <<< 8) ||| int rsaParameters.Exponent[i]
+            match rsaParameters.Exponent with
+            | null -> ()
+            | exp ->
+                for i in 0 .. exp.Length - 1 do
+                    buffer <- (buffer <<< 8) ||| int exp[i]
 
             buffer
 
+        let safeArrayRev (buffer: _ MaybeNull) =
+            match buffer with
+            | Null -> Array.empty<byte>
+            | NonNull buffer -> buffer |> Array.rev
+
         bw.Write expAsDword // RSAPubKey.pubExp
-        bw.Write(rsaParameters.Modulus |> Array.rev) // Copy over the modulus for both public and private
+        bw.Write(rsaParameters.Modulus |> safeArrayRev) // Copy over the modulus for both public and private
 
         if isPrivate then
             do
-                bw.Write(rsaParameters.P |> Array.rev)
-                bw.Write(rsaParameters.Q |> Array.rev)
-                bw.Write(rsaParameters.DP |> Array.rev)
-                bw.Write(rsaParameters.DQ |> Array.rev)
-                bw.Write(rsaParameters.InverseQ |> Array.rev)
-                bw.Write(rsaParameters.D |> Array.rev)
+                bw.Write(rsaParameters.P |> safeArrayRev)
+                bw.Write(rsaParameters.Q |> safeArrayRev)
+                bw.Write(rsaParameters.DP |> safeArrayRev)
+                bw.Write(rsaParameters.DQ |> safeArrayRev)
+                bw.Write(rsaParameters.InverseQ |> safeArrayRev)
+                bw.Write(rsaParameters.D |> safeArrayRev)
 
         bw.Flush()
         ms.ToArray()
 
     key
 
-let createSignature hash keyBlob keyType =
+let createSignature (hash: byte array) keyBlob keyType =
     use rsa = RSA.Create()
     rsa.ImportParameters(RSAParamatersFromBlob keyBlob keyType)
 
@@ -260,7 +272,8 @@ let createSignature hash keyBlob keyType =
 
 let patchSignature (stream: Stream) (peReader: PEReader) (signature: byte array) =
     let peHeaders = peReader.PEHeaders
-    let signatureDirectory = peHeaders.CorHeader.StrongNameSignatureDirectory
+    let corHeader = peHeaders.CorHeader |> nullArgCheck (nameof peHeaders.CorHeader)
+    let signatureDirectory = corHeader.StrongNameSignatureDirectory
 
     let signatureOffset =
         if signatureDirectory.Size > signature.Length then
@@ -275,7 +288,7 @@ let patchSignature (stream: Stream) (peReader: PEReader) (signature: byte array)
 
     let corHeaderFlagsOffset = int64 (peHeaders.CorHeaderStartOffset + 16) // offsetof(IMAGE_COR20_HEADER, Flags)
     stream.Seek(corHeaderFlagsOffset, SeekOrigin.Begin) |> ignore
-    stream.WriteByte(byte (peHeaders.CorHeader.Flags ||| CorFlags.StrongNameSigned))
+    stream.WriteByte(byte (corHeader.Flags ||| CorFlags.StrongNameSigned))
     ()
 
 let signStream stream keyBlob =

src/Compiler/AbstractIL/ilwrite.fs

@@ -362,7 +362,11 @@ let envForOverrideSpec (ospec: ILOverridesSpec) = { EnclosingTyparCount=ospec.De
 //---------------------------------------------------------------------
 
 [<NoEquality; NoComparison>]
-type MetadataTable<'T> =
+type MetadataTable<'T
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    > =
     { name: string
       dict: Dictionary<'T, int> // given a row, find its entry number
       mutable rows: ResizeArray<'T> }

src/Compiler/AbstractIL/ilwritepdb.fs

@@ -343,10 +343,7 @@ let scopeSorter (scope1: PdbMethodScope) (scope2: PdbMethodScope) =
 type PortablePdbGenerator
     (embedAllSource: bool, embedSourceList: string list, sourceLink: string, checksumAlgorithm, info: PdbData, pathMap: PathMap) =
 
-    let docs =
-        match info.Documents with
-        | Null -> Array.empty
-        | NonNull docs -> docs
+    let docs = info.Documents
 
     // The metadata to wite to the PoortablePDB (Roslyn = _debugMetadataOpt)
 
@@ -393,7 +390,7 @@ type PortablePdbGenerator
     /// </summary>
     let sourceCompressionThreshold = 200
 
-    let includeSource file =
+    let includeSource (file: string) =
         let isInList =
             embedSourceList
             |> List.exists (fun f -> String.Compare(file, f, StringComparison.OrdinalIgnoreCase) = 0)
@@ -654,12 +651,9 @@ type PortablePdbGenerator
     let emitMethod minfo =
         let docHandle, sequencePointBlob =
             let sps =
-                match minfo.DebugPoints with
-                | Null -> Array.empty
-                | NonNull pts ->
-                    match minfo.DebugRange with
-                    | None -> Array.empty
-                    | Some _ -> pts
+                match minfo.DebugRange with
+                | None -> Array.empty
+                | Some _ -> minfo.DebugPoints
 
             let builder = BlobBuilder()
             builder.WriteCompressedInteger(minfo.LocalSignatureToken)
@@ -872,7 +866,7 @@ let getInfoForEmbeddedPortablePdb
     (uncompressedLength: int64)
     (contentId: BlobContentId)
     (compressedStream: MemoryStream)
-    pdbfile
+    (pdbfile: string)
     cvChunk
     pdbChunk
     deterministicPdbChunk
@@ -886,7 +880,7 @@ let getInfoForEmbeddedPortablePdb
     pdbGetDebugInfo
         (contentId.Guid.ToByteArray())
         (int32 contentId.Stamp)
-        fn
+        !!fn
         cvChunk
         (Some pdbChunk)
         deterministicPdbChunk

src/Compiler/Checking/AttributeChecking.fs

@@ -380,7 +380,7 @@ let CheckFSharpAttributesForUnseen g attribs _m =
 #if !NO_TYPEPROVIDERS
 /// Indicate if a list of provided attributes contains 'ObsoleteAttribute'. Used to suppress the item in intellisense.
 let CheckProvidedAttributesForUnseen (provAttribs: Tainted<IProvidedCustomAttributeProvider>) m = 
-    provAttribs.PUntaint((fun a -> a.GetAttributeConstructorArgs(provAttribs.TypeProvider.PUntaintNoFailure(id), typeof<ObsoleteAttribute>.FullName).IsSome), m)
+    provAttribs.PUntaint((fun a -> a.GetAttributeConstructorArgs(provAttribs.TypeProvider.PUntaintNoFailure(id), !! typeof<ObsoleteAttribute>.FullName).IsSome), m)
 #endif
 
 /// Check the attributes associated with a property, returning warnings and errors as data.
@@ -479,7 +479,7 @@ let MethInfoIsUnseen g (m: range) (ty: TType) minfo =
         // just to look at the attributes on IL methods.
         if tcref.IsILTycon then 
                 tcref.ILTyconRawMetadata.CustomAttrs.AsArray()
-                |> Array.exists (fun attr -> attr.Method.DeclaringType.TypeSpec.Name = typeof<TypeProviderEditorHideMethodsAttribute>.FullName)
+                |> Array.exists (fun attr -> attr.Method.DeclaringType.TypeSpec.Name = !! typeof<TypeProviderEditorHideMethodsAttribute>.FullName)
         else 
             false
 #else

src/Compiler/Checking/CheckFormatStrings.fs

@@ -359,10 +359,10 @@ let parseFormatStringInternal
             let acc = if widthArg then (Option.map ((+)1) posi, g.int_ty) :: acc else acc
 
             let checkOtherFlags c =
-                if info.precision then failwith (FSComp.SR.forFormatDoesntSupportPrecision(c.ToString()))
-                if info.addZeros then failwith (FSComp.SR.forDoesNotSupportZeroFlag(c.ToString()))
+                if info.precision then failwith (FSComp.SR.forFormatDoesntSupportPrecision(c.ToString() |> string))
+                if info.addZeros then failwith (FSComp.SR.forDoesNotSupportZeroFlag(c.ToString() |> string))
                 match info.numPrefixIfPos with
-                | Some n -> failwith (FSComp.SR.forDoesNotSupportPrefixFlag(c.ToString(), n.ToString()))
+                | Some n -> failwith (FSComp.SR.forDoesNotSupportPrefixFlag(c.ToString() |> string, n.ToString()))
                 | None -> ()
 
             let skipPossibleInterpolationHole pos = Parse.skipPossibleInterpolationHole isInterpolated isFormattableString fmt pos

src/Compiler/Checking/CheckPatterns.fs

@@ -655,8 +655,8 @@ and TcPatLongIdentUnionCaseOrExnCase warnOnUpper cenv env ad vFlags patEnv ty (m
                     CallNameResolutionSink cenv.tcSink (id.idRange, env.NameEnv, argItem, emptyTyparInst, ItemOccurence.Pattern, ad)
 
                     match box result[idx] with
-                    | Null -> result[idx] <- pat
-                    | NonNull _ ->
+                    | null -> result[idx] <- pat
+                    | _ ->
                         extraPatterns.Add pat
                         errorR (Error (FSComp.SR.tcUnionCaseFieldCannotBeUsedMoreThanOnce id.idText, id.idRange))
 

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -4913,8 +4913,8 @@ and CrackStaticConstantArgs (cenv: cenv) env tpenv (staticParameters: Tainted<Pr
                 | [] ->
                     if sp.PUntaint((fun sp -> sp.IsOptional), m) then
                          match sp.PUntaint((fun sp -> sp.RawDefaultValue), m) with
-                         | Null -> error (Error(FSComp.SR.etStaticParameterRequiresAValue (spName, containerName, containerName, spName), m))
-                         | NonNull v -> v
+                         | null -> error (Error(FSComp.SR.etStaticParameterRequiresAValue (spName, containerName, containerName, spName), m))
+                         | v -> v
                     else
                       error (Error(FSComp.SR.etStaticParameterRequiresAValue (spName, containerName, containerName, spName), m))
                  | ps ->

src/Compiler/Checking/InfoReader.fs

@@ -731,25 +731,28 @@ type InfoReader(g: TcGlobals, amap: Import.ImportMap) as this =
               // Only cache closed, monomorphic types (closed = all members for the type
               // have been processed). Generic type instantiations could be processed if we had 
               // a decent hash function for these.
+
+              // Nullness of `ty` (TType_app) is not considered here, as the info is used to load members of the type
+              // It would matter for different generic instantiations of the same type, but we don't cache that here - TType_app is always matched for `[]` typars.
               canMemoize=(fun (_flags, _: range, ty) -> 
                                     match stripTyEqns g ty with 
-                                    | TType_app(tcref, [], _) -> tcref.TypeContents.tcaug_closed // TODO NULLNESS: consider whether ignoring _nullness is valid here
+                                    | TType_app(tcref, [], _) -> tcref.TypeContents.tcaug_closed
                                     | _ -> false),
               
               keyComparer=
                  { new IEqualityComparer<_> with 
-                       member _.Equals((flags1, _, ty1), (flags2, _, ty2)) =
-                                    // Ignoring the ranges - that's OK.
-                                    flagsEq.Equals(flags1, flags2) && 
-                                    match stripTyEqns g ty1, stripTyEqns g ty2 with 
-                                    | TType_app(tcref1, [], _),TType_app(tcref2, [], _) -> tyconRefEq g tcref1 tcref2  // TODO NULLNESS: consider whether ignoring _nullness is valid here
-                                    | _ -> false
                        member _.GetHashCode((flags, _, ty)) =
                                     // Ignoring the ranges - that's OK.
                                     flagsEq.GetHashCode flags + 
                                     (match stripTyEqns g ty with 
-                                     | TType_app(tcref, [], _) -> hash tcref.LogicalName  // TODO NULLNESS: consider whether ignoring _nullness is valid here
-                                     | _ -> 0) })
+                                     | TType_app(tcref, [], _) -> hash tcref.LogicalName
+                                     | _ -> 0)
+                       member _.Equals((flags1, _, ty1), (flags2, _, ty2)) =
+                                    // Ignoring the ranges - that's OK.
+                                    flagsEq.Equals(flags1, flags2) && 
+                                    match stripTyEqns g ty1, stripTyEqns g ty2 with 
+                                    | TType_app(tcref1, [], _),TType_app(tcref2, [], _) -> tyconRefEq g tcref1 tcref2
+                                    | _ -> false })
     
     let FindImplicitConversionsUncached (ad, m, ty) = 
         if isTyparTy g ty then 
@@ -791,7 +794,7 @@ type InfoReader(g: TcGlobals, amap: Import.ImportMap) as this =
     let hashFlags3 = 
         { new IEqualityComparer<AccessorDomain> with 
                member _.GetHashCode((ad: AccessorDomain)) = AccessorDomain.CustomGetHashCode ad
-               member _.Equals((ad1), (ad2)) = AccessorDomain.CustomEquals(g, ad1, ad2) }
+               member _.Equals((ad1), (ad2)) = nullSafeEquality ad1 ad2 (fun ad1 ad2 -> AccessorDomain.CustomEquals(g, ad1, ad2)) }
                          
     let hashFlags4 = 
         { new IEqualityComparer<AccessorDomain * string> with 

src/Compiler/Checking/MethodCalls.fs

@@ -1727,7 +1727,7 @@ let AdjustCallerArgs tcVal tcFieldInit eCallerMemberName (infoReader: InfoReader
 // This file is not a great place for this functionality to sit, it's here because of BuildMethodCall
 module ProvidedMethodCalls =
 
-    let private convertConstExpr g amap m (constant : Tainted<obj * ProvidedType>) =
+    let private convertConstExpr g amap m (constant : Tainted<objnull * ProvidedType>) =
         let obj, objTy = constant.PApply2(id, m)
         let ty = Import.ImportProvidedType amap m objTy
         let normTy = normalizeEnumTy g ty

src/Compiler/Checking/NameResolution.fs

@@ -4296,7 +4296,7 @@ let ItemOfTy g x =
     Item.Types (nm, [x])
 
 // Filter out 'PrivateImplementationDetail' classes
-let IsInterestingModuleName nm = not (System.String.IsNullOrEmpty nm) && nm[0] <> '<'
+let IsInterestingModuleName nm = not (System.String.IsNullOrEmpty nm) && (!!nm)[0] <> '<'
 
 let rec PartialResolveLookupInModuleOrNamespaceAsModuleOrNamespaceThen f plid (modref: ModuleOrNamespaceRef) =
     let mty = modref.ModuleOrNamespaceType

src/Compiler/Checking/NicePrint.fs

@@ -263,7 +263,7 @@ module internal PrintUtilities =
             if possibleXmlDoc.IsEmpty then
                 match info with
                 | Some(Some ccuFileName, xmlDocSig) ->
-                    infoReader.amap.assemblyLoader.TryFindXmlDocumentationInfo(Path.GetFileNameWithoutExtension ccuFileName)
+                    infoReader.amap.assemblyLoader.TryFindXmlDocumentationInfo(!!Path.GetFileNameWithoutExtension(ccuFileName))
                     |> Option.bind (fun xmlDocInfo ->
                         xmlDocInfo.TryGetXmlDocBySig(xmlDocSig)
                     )
@@ -2913,7 +2913,7 @@ let minimalStringsOfTwoTypes denv ty1 ty2 =
         let denv = denv.SetOpenPaths []
         let denv = { denv with includeStaticParametersInTypeNames=true }
         let makeName t =
-            let assemblyName = PrintTypes.layoutAssemblyName denv t |> function null | "" -> "" | name -> sprintf " (%s)" name
+            let assemblyName = PrintTypes.layoutAssemblyName denv t |> function Null | NonNull "" -> "" | NonNull name -> sprintf " (%s)" name
             sprintf "%s%s" (stringOfTy denv t) assemblyName
 
         (makeName ty1, makeName ty2, stringOfTyparConstraints denv tpcs)

src/Compiler/Checking/PostInferenceChecks.fs

@@ -2345,25 +2345,25 @@ let CheckEntityDefn cenv env (tycon: Entity) =
                     ignore isInArg
                     match (optArgInfo, callerInfo) with
                     | _, NoCallerInfo -> ()
-                    | NotOptional, _ -> errorR(Error(FSComp.SR.tcCallerInfoNotOptional(callerInfo.ToString()), m))
+                    | NotOptional, _ -> errorR(Error(FSComp.SR.tcCallerInfoNotOptional(callerInfo |> string), m))
                     | CallerSide _, CallerLineNumber ->
                         if not (typeEquiv g g.int32_ty ty) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "int", NicePrint.minimalStringOfType cenv.denv ty), m))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "int", NicePrint.minimalStringOfType cenv.denv ty), m))
                     | CalleeSide, CallerLineNumber ->
                         if not ((isOptionTy g ty) && (typeEquiv g g.int32_ty (destOptionTy g ty))) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "int", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "int", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m))
                     | CallerSide _, CallerFilePath ->
                         if not (typeEquiv g g.string_ty ty) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "string", NicePrint.minimalStringOfType cenv.denv ty), m))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "string", NicePrint.minimalStringOfType cenv.denv ty), m))
                     | CalleeSide, CallerFilePath ->
                         if not ((isOptionTy g ty) && (typeEquiv g g.string_ty (destOptionTy g ty))) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "string", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "string", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m))
                     | CallerSide _, CallerMemberName ->
                         if not (typeEquiv g g.string_ty ty) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "string", NicePrint.minimalStringOfType cenv.denv ty), m))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "string", NicePrint.minimalStringOfType cenv.denv ty), m))
                     | CalleeSide, CallerMemberName ->
                         if not ((isOptionTy g ty) && (typeEquiv g g.string_ty (destOptionTy g ty))) then
-                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo.ToString(), "string", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m)))
+                            errorR(Error(FSComp.SR.tcCallerInfoWrongType(callerInfo |> string, "string", NicePrint.minimalStringOfType cenv.denv (destOptionTy g ty)), m)))
 
         for pinfo in immediateProps do
             let nm = pinfo.PropertyName

src/Compiler/Checking/TypeHierarchy.fs

@@ -47,7 +47,7 @@ let GetSuperTypeOfType g amap m ty =
 #if !NO_TYPEPROVIDERS
         | ProvidedTypeMetadata info ->
             let st = info.ProvidedType
-            let superOpt = st.PApplyOption((fun st -> match st.BaseType with null -> None | t -> Some (nonNull t)), m)
+            let superOpt = st.PApplyOption((fun st -> match st.BaseType with null -> None | t -> Some t), m)
             match superOpt with
             | None -> None
             | Some super -> Some(ImportProvidedType amap m super)

src/Compiler/Checking/infos.fs

@@ -344,12 +344,12 @@ let CrackParamAttribsInfo g (ty: TType, argInfo: ArgReprInfo) =
 type ILFieldInit with
 
     /// Compute the ILFieldInit for the given provided constant value for a provided enum type.
-    static member FromProvidedObj m (v: obj) =
+    static member FromProvidedObj m (v: obj MaybeNull) =
         match v with
         | Null -> ILFieldInit.Null
         | NonNull v ->
             let objTy = v.GetType()
-            let v = if objTy.IsEnum then objTy.GetField("value__").GetValue v else v
+            let v = if objTy.IsEnum then !!(!!objTy.GetField("value__")).GetValue v else v
             match v with
             | :? single as i -> ILFieldInit.Single i
             | :? double as i -> ILFieldInit.Double i
@@ -364,7 +364,7 @@ type ILFieldInit with
             | :? uint32 as i -> ILFieldInit.UInt32 i
             | :? int64 as i -> ILFieldInit.Int64 i
             | :? uint64 as i -> ILFieldInit.UInt64 i
-            | _ -> error(Error(FSComp.SR.infosInvalidProvidedLiteralValue(try v.ToString() with _ -> "?"), m))
+            | _ -> error(Error(FSComp.SR.infosInvalidProvidedLiteralValue(try !!v.ToString() with _ -> "?"), m))
 
 
 /// Compute the OptionalArgInfo for a provided parameter.
@@ -1259,10 +1259,10 @@ type MethInfo =
         | ProvidedMeth(amap, mi, _, _) ->
             // A single group of tupled arguments
             [ [for p in mi.PApplyArray((fun mi -> mi.GetParameters()), "GetParameters", m) do
-                let isParamArrayArg = p.PUntaint((fun px -> (px :> IProvidedCustomAttributeProvider).GetAttributeConstructorArgs(p.TypeProvider.PUntaintNoFailure id, typeof<ParamArrayAttribute>.FullName).IsSome), m)
+                let isParamArrayArg = p.PUntaint((fun px -> (px :> IProvidedCustomAttributeProvider).GetAttributeConstructorArgs(p.TypeProvider.PUntaintNoFailure id, !! typeof<ParamArrayAttribute>.FullName).IsSome), m)
                 let optArgInfo =  OptionalArgInfoOfProvidedParameter amap m p
                 let reflArgInfo =
-                    match p.PUntaint((fun px -> (px :> IProvidedCustomAttributeProvider).GetAttributeConstructorArgs(p.TypeProvider.PUntaintNoFailure id, typeof<ReflectedDefinitionAttribute>.FullName)), m) with
+                    match p.PUntaint((fun px -> (px :> IProvidedCustomAttributeProvider).GetAttributeConstructorArgs(p.TypeProvider.PUntaintNoFailure id, !! typeof<ReflectedDefinitionAttribute>.FullName)), m) with
                     | Some ([ Some (:? bool as b) ], _) -> ReflectedArgInfo.Quote b
                     | Some _ -> ReflectedArgInfo.Quote false
                     | None -> ReflectedArgInfo.None
@@ -1726,7 +1726,7 @@ type ILPropInfo =
         let nullness = {DirectAttributes = AttributesFromIL(pdef.MetadataIndex,pdef.CustomAttrsStored); Fallback = tinfo.NullableClassSource}
         ImportILTypeFromMetadata amap m tinfo.ILScopeRef tinfo.TypeInstOfRawMetadata [] nullness pdef.PropertyType
 
-    override x.ToString() = x.ILTypeInfo.ToString() + "::" + x.PropertyName
+    override x.ToString() = !!x.ILTypeInfo.ToString() + "::" + x.PropertyName
 
 /// Describes an F# use of a property
 [<NoComparison; NoEquality>]
@@ -2184,7 +2184,7 @@ type ILEventInfo =
     /// Indicates if the property is static
     member x.IsStatic = x.AddMethod.IsStatic
 
-    override x.ToString() = x.ILTypeInfo.ToString() + "::" + x.EventName
+    override x.ToString() = !!x.ILTypeInfo.ToString() + "::" + x.EventName
 
 //-------------------------------------------------------------------------
 // Helpers for EventInfo

src/Compiler/CodeGen/IlxGen.fs

@@ -12078,14 +12078,14 @@ let LookupGeneratedValue (cenv: cenv) (ctxt: ExecutionContext) eenv (v: Val) =
                 if hasLiteralAttr then
                     let staticTy = ctxt.LookupTypeRef fspec.DeclaringTypeRef
                     // Checked: This FieldInfo (FieldBuilder) supports GetValue().
-                    staticTy.GetField(fspec.Name).GetValue(null: obj)
+                    (!! staticTy.GetField(fspec.Name)).GetValue(null: obj MaybeNull)
                 else
                     let staticTy = ctxt.LookupTypeRef ilContainerTy.TypeRef
                     // We can't call .Invoke on the ILMethodRef's MethodInfo,
                     // because it is the MethodBuilder and that does not support Invoke.
                     // Rather, we look for the getter MethodInfo from the built type and .Invoke on that.
                     let methInfo =
-                        staticTy.GetMethod(ilGetterMethRef.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
+                        !! staticTy.GetMethod(ilGetterMethRef.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
 
                     methInfo.Invoke(null, null)
 
@@ -12098,7 +12098,7 @@ let LookupGeneratedValue (cenv: cenv) (ctxt: ExecutionContext) eenv (v: Val) =
                 // because it is the MethodBuilder and that does not support Invoke.
                 // Rather, we look for the getter MethodInfo from the built type and .Invoke on that.
                 let methInfo =
-                    staticTy.GetMethod(ilGetterMethSpec.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
+                    !! staticTy.GetMethod(ilGetterMethSpec.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
 
                 methInfo.Invoke(null, null)
 
@@ -12125,14 +12125,14 @@ let SetGeneratedValue (ctxt: ExecutionContext) eenv isForced (v: Val) (value: ob
                     let staticTy = ctxt.LookupTypeRef fspec.DeclaringTypeRef
 
                     let fieldInfo =
-                        staticTy.GetField(fspec.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
+                        !! staticTy.GetField(fspec.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
 
                     fieldInfo.SetValue(null, value)
                 else
                     let staticTy = ctxt.LookupTypeRef ilSetterMethRef.DeclaringTypeRef
 
                     let methInfo =
-                        staticTy.GetMethod(ilSetterMethRef.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
+                        !! staticTy.GetMethod(ilSetterMethRef.Name, BindingFlags.Static ||| BindingFlags.Public ||| BindingFlags.NonPublic)
 
                     methInfo.Invoke(null, [| value |]) |> ignore
         | _ -> ()

src/Compiler/DependencyManager/AssemblyResolveHandler.fs

@@ -6,6 +6,7 @@ open System
 open System.IO
 open System.Reflection
 open Internal.Utilities.FSharpEnvironment
+open Internal.Utilities.Library
 
 /// Signature for ResolutionProbe callback
 /// host implements this, it's job is to return a list of assembly paths to probe.
@@ -14,25 +15,24 @@ type AssemblyResolutionProbe = delegate of Unit -> seq<string>
 /// Type that encapsulates AssemblyResolveHandler for managed packages
 type AssemblyResolveHandlerCoreclr(assemblyProbingPaths: AssemblyResolutionProbe option) as this =
     let loadContextType =
-        Type.GetType("System.Runtime.Loader.AssemblyLoadContext, System.Runtime.Loader", false)
+        !! Type.GetType("System.Runtime.Loader.AssemblyLoadContext, System.Runtime.Loader", false)
 
     let loadFromAssemblyPathMethod =
-        loadContextType.GetMethod("LoadFromAssemblyPath", [| typeof<string> |])
+        !! loadContextType.GetMethod("LoadFromAssemblyPath", [| typeof<string> |])
 
-    let eventInfo = loadContextType.GetEvent("Resolving")
+    let eventInfo = !! loadContextType.GetEvent("Resolving")
 
     let handler, defaultAssemblyLoadContext =
         let ti = typeof<AssemblyResolveHandlerCoreclr>
 
         let gmi =
-            ti.GetMethod("ResolveAssemblyNetStandard", BindingFlags.Instance ||| BindingFlags.NonPublic)
+            !! ti.GetMethod("ResolveAssemblyNetStandard", BindingFlags.Instance ||| BindingFlags.NonPublic)
 
         let mi = gmi.MakeGenericMethod(loadContextType)
-        let del = Delegate.CreateDelegate(eventInfo.EventHandlerType, this, mi)
+        let del = Delegate.CreateDelegate(!!eventInfo.EventHandlerType, this, mi)
 
         let prop =
-            loadContextType
-                .GetProperty("Default", BindingFlags.Static ||| BindingFlags.Public)
+            (!! loadContextType.GetProperty("Default", BindingFlags.Static ||| BindingFlags.Public))
                 .GetValue(null, null)
 
         del, prop
@@ -113,7 +113,7 @@ type AssemblyResolveHandler internal (assemblyProbingPaths: AssemblyResolutionPr
             else
                 new AssemblyResolveHandlerDeskTop(assemblyProbingPaths) :> IDisposable)
 
-    new(assemblyProbingPaths: AssemblyResolutionProbe) = new AssemblyResolveHandler(Option.ofObj assemblyProbingPaths)
+    new(assemblyProbingPaths: AssemblyResolutionProbe MaybeNull) = new AssemblyResolveHandler(Option.ofObj assemblyProbingPaths)
 
     interface IDisposable with
         member _.Dispose() =

src/Compiler/DependencyManager/DependencyProvider.fs

@@ -15,8 +15,11 @@ open System.Collections.Concurrent
 module Option =
 
     /// Convert string into Option string where null and String.Empty result in None
-    let ofString s =
-        if String.IsNullOrEmpty(s) then None else Some(s)
+    let ofString (s: string MaybeNull) =
+        match s with
+        | null -> None
+        | "" -> None
+        | s -> Some s
 
 [<AutoOpen>]
 module ReflectionHelper =
@@ -57,31 +60,27 @@ module ReflectionHelper =
             let instanceFlags =
                 BindingFlags.Public ||| BindingFlags.NonPublic ||| BindingFlags.Instance
 
-            let property =
-                theType.GetProperty(propertyName, instanceFlags, null, typeof<'T>, [||], [||])
-
-            if isNull property then
-                None
-            else
-                let getMethod = property.GetGetMethod()
-
-                if not (isNull getMethod) && not getMethod.IsStatic then
-                    Some property
-                else
-                    None
+            match theType.GetProperty(propertyName, instanceFlags, null, typeof<'T>, [||], [||]) with
+            | null -> None
+            | property ->
+                match property.GetGetMethod() with
+                | null -> None
+                | getMethod when getMethod.IsStatic -> None
+                | _ -> Some property
         with _ ->
             None
 
     let getInstanceMethod<'T> (theType: Type) (parameterTypes: Type[]) methodName =
         try
-            let theMethod = theType.GetMethod(methodName, parameterTypes)
-            if isNull theMethod then None else Some theMethod
+            match theType.GetMethod(methodName, parameterTypes) with
+            | null -> None
+            | theMethod -> Some theMethod
         with _ ->
             None
 
     let stripTieWrapper (e: Exception) =
         match e with
-        | :? TargetInvocationException as e -> e.InnerException
+        | :? TargetInvocationException as e when isNotNull e.InnerException -> !!e.InnerException
         | _ -> e
 
 /// Indicate the type of error to report
@@ -500,7 +499,7 @@ type DependencyProvider
                 let assemblyLocation =
                     typeof<IDependencyManagerProvider>.GetTypeInfo().Assembly.Location
 
-                yield Path.GetDirectoryName assemblyLocation
+                yield !!(Path.GetDirectoryName assemblyLocation)
                 yield AppDomain.CurrentDomain.BaseDirectory
             ])
 

src/Compiler/DependencyManager/NativeDllResolveHandler.fs

@@ -24,12 +24,12 @@ type internal ProbingPathsStore() =
         else
             p
 
-    static member RemoveProbeFromProcessPath probePath =
+    static member RemoveProbeFromProcessPath(probePath: string) =
         if not (String.IsNullOrWhiteSpace(probePath)) then
             let probe = ProbingPathsStore.AppendPathSeparator probePath
 
             let path =
-                ProbingPathsStore.AppendPathSeparator(Environment.GetEnvironmentVariable("PATH"))
+                ProbingPathsStore.AppendPathSeparator(Environment.GetEnvironmentVariable("PATH") |> defaultIfNull "")
 
             if path.Contains(probe) then
                 Environment.SetEnvironmentVariable("PATH", path.Replace(probe, ""))
@@ -38,7 +38,7 @@ type internal ProbingPathsStore() =
         let probe = ProbingPathsStore.AppendPathSeparator probePath
 
         let path =
-            ProbingPathsStore.AppendPathSeparator(Environment.GetEnvironmentVariable("PATH"))
+            ProbingPathsStore.AppendPathSeparator(Environment.GetEnvironmentVariable("PATH") |> defaultIfNull "")
 
         if not (path.Contains(probe)) then
             Environment.SetEnvironmentVariable("PATH", path + probe)
@@ -72,9 +72,9 @@ type internal NativeDllResolveHandlerCoreClr(nativeProbingRoots: NativeResolutio
 
     let nativeLibraryTryLoad =
         let nativeLibraryType: Type =
-            Type.GetType("System.Runtime.InteropServices.NativeLibrary, System.Runtime.InteropServices", false)
+            !! Type.GetType("System.Runtime.InteropServices.NativeLibrary, System.Runtime.InteropServices", false)
 
-        nativeLibraryType.GetMethod("TryLoad", [| typeof<string>; typeof<IntPtr>.MakeByRefType() |])
+        !! nativeLibraryType.GetMethod("TryLoad", [| typeof<string>; typeof<IntPtr>.MakeByRefType() |])
 
     let loadNativeLibrary path =
         let arguments = [| path :> obj; IntPtr.Zero :> obj |]
@@ -157,13 +157,12 @@ type internal NativeDllResolveHandlerCoreClr(nativeProbingRoots: NativeResolutio
     // netstandard 2.1 has this property, unfortunately we don't build with that yet
     //public event Func<Assembly, string, IntPtr> ResolvingUnmanagedDll
     let assemblyLoadContextType: Type =
-        Type.GetType("System.Runtime.Loader.AssemblyLoadContext, System.Runtime.Loader", false)
+        !! Type.GetType("System.Runtime.Loader.AssemblyLoadContext, System.Runtime.Loader", false)
 
     let eventInfo, handler, defaultAssemblyLoadContext =
-        assemblyLoadContextType.GetEvent("ResolvingUnmanagedDll"),
+        !! assemblyLoadContextType.GetEvent("ResolvingUnmanagedDll"),
         Func<Assembly, string, IntPtr> resolveUnmanagedDll,
-        assemblyLoadContextType
-            .GetProperty("Default", BindingFlags.Static ||| BindingFlags.Public)
+        (!! assemblyLoadContextType.GetProperty("Default", BindingFlags.Static ||| BindingFlags.Public))
             .GetValue(null, null)
 
     do eventInfo.AddEventHandler(defaultAssemblyLoadContext, handler)
@@ -185,7 +184,7 @@ type NativeDllResolveHandler(nativeProbingRoots: NativeResolutionProbe option) =
         |> Option.filter (fun _ -> isRunningOnCoreClr)
         |> Option.map (fun _ -> new NativeDllResolveHandlerCoreClr(nativeProbingRoots))
 
-    new(nativeProbingRoots: NativeResolutionProbe) = new NativeDllResolveHandler(Option.ofObj nativeProbingRoots)
+    new(nativeProbingRoots: NativeResolutionProbe MaybeNull) = new NativeDllResolveHandler(Option.ofObj nativeProbingRoots)
 
     member internal _.RefreshPathsInEnvironment(roots: string seq) =
         handler |> Option.iter (fun handler -> handler.RefreshPathsInEnvironment(roots))

src/Compiler/Driver/CompilerConfig.fs

@@ -180,7 +180,7 @@ type VersionFlag =
             else
                 use fs = FileSystem.OpenFileForReadShim(s)
                 use is = new StreamReader(fs)
-                is.ReadLine()
+                !! is.ReadLine()
         | VersionNone -> "0.0.0.0"
 
 /// Represents a reference to an assembly. May be backed by a real assembly on disk, or a cross-project
@@ -635,7 +635,11 @@ type TcConfigBuilder =
         seq {
             yield! tcConfigB.includes
             yield! tcConfigB.compilerToolPaths
-            yield! (tcConfigB.referencedDLLs |> Seq.map (fun ref -> Path.GetDirectoryName(ref.Text)))
+
+            yield!
+                (tcConfigB.referencedDLLs
+                 |> Seq.map (fun ref -> !! Path.GetDirectoryName(ref.Text)))
+
             tcConfigB.implicitIncludeDir
         }
         |> Seq.distinct
@@ -654,8 +658,8 @@ type TcConfigBuilder =
             rangeForErrors
         ) =
 
-        if (String.IsNullOrEmpty defaultFSharpBinariesDir) then
-            failwith "Expected a valid defaultFSharpBinariesDir"
+        let defaultFSharpBinariesDir =
+            nullArgCheck "defaultFSharpBinariesDir" defaultFSharpBinariesDir
 
         // These are all default values, many can be overridden using the command line switch
         {
@@ -1107,7 +1111,7 @@ type TcConfig private (data: TcConfigBuilder, validate: bool) =
     // clone the input builder to ensure nobody messes with it.
     let data = { data with pause = data.pause }
 
-    let computeKnownDllReference libraryName =
+    let computeKnownDllReference (libraryName: string) =
         let defaultCoreLibraryReference =
             AssemblyReference(range0, libraryName + ".dll", None)
 
@@ -1159,7 +1163,7 @@ type TcConfig private (data: TcConfigBuilder, validate: bool) =
                 ComputeMakePathAbsolute data.implicitIncludeDir primaryAssemblyFilename
 
             try
-                let clrRoot = Some(Path.GetDirectoryName(FileSystem.GetFullPathShim fileName))
+                let clrRoot = Some(!! Path.GetDirectoryName(FileSystem.GetFullPathShim fileName))
                 clrRoot, data.legacyReferenceResolver.Impl.HighestInstalledNetFrameworkVersion()
             with e ->
                 // We no longer expect the above to fail but leaving this just in case
@@ -1459,7 +1463,7 @@ type TcConfig private (data: TcConfigBuilder, validate: bool) =
     /// 'framework' reference set that is potentially shared across multiple compilations.
     member tcConfig.IsSystemAssembly(fileName: string) =
         try
-            let dirName = Path.GetDirectoryName fileName
+            let dirName = !! Path.GetDirectoryName(fileName)
             let baseName = FileSystemUtils.fileNameWithoutExtension fileName
 
             FileSystem.FileExistsShim fileName

src/Compiler/Driver/CompilerDiagnostics.fs

@@ -210,7 +210,7 @@ type Exception with
         | HashLoadedSourceHasIssues(_, _, _, m)
         | HashLoadedScriptConsideredSource m -> Some m
         // Strip TargetInvocationException wrappers
-        | :? System.Reflection.TargetInvocationException as e -> e.InnerException.DiagnosticRange
+        | :? System.Reflection.TargetInvocationException as e when isNotNull e.InnerException -> (!!e.InnerException).DiagnosticRange
 #if !NO_TYPEPROVIDERS
         | :? TypeProviderError as e -> e.Range |> Some
 #endif
@@ -338,7 +338,7 @@ type Exception with
         | ArgumentsInSigAndImplMismatch _ -> 3218
 
         // Strip TargetInvocationException wrappers
-        | :? TargetInvocationException as e -> e.InnerException.DiagnosticNumber
+        | :? TargetInvocationException as e when isNotNull e.InnerException -> (!!e.InnerException).DiagnosticNumber
         | WrappedError(e, _) -> e.DiagnosticNumber
         | DiagnosticWithText(n, _, _) -> n
         | DiagnosticWithSuggestions(n, _, _, _, _) -> n
@@ -1945,7 +1945,7 @@ type Exception with
             )
 
         // Strip TargetInvocationException wrappers
-        | :? TargetInvocationException as exn -> exn.InnerException.Output(os, suggestNames)
+        | :? TargetInvocationException as e when isNotNull e.InnerException -> (!!e.InnerException).Output(os, suggestNames)
 
         | :? FileNotFoundException as exn -> Printf.bprintf os "%s" exn.Message
 

src/Compiler/Driver/CompilerImports.fs

@@ -163,7 +163,7 @@ let PickleToResource inMem file (g: TcGlobals) compress scope rName rNameB p x =
     let byteStorage = ByteStorage.FromByteArray(bytes)
 
     let byteStorageB =
-        if inMem then
+        if inMem then            
             ByteStorage.FromMemoryAndCopy(bytesB.AsMemory(), useBackingMemoryMappedFile = true)
         else
             ByteStorage.FromByteArray(bytesB.AsMemory().ToArray())
@@ -478,15 +478,15 @@ let isHashRReference (r: range) =
     && not (equals r rangeCmdArgs)
     && FileSystem.IsPathRootedShim r.FileName
 
-let IsNetModule fileName =
+let IsNetModule (fileName:string) =
     let ext = Path.GetExtension fileName
     String.Compare(ext, ".netmodule", StringComparison.OrdinalIgnoreCase) = 0
 
-let IsDLL fileName =
+let IsDLL (fileName:string) =
     let ext = Path.GetExtension fileName
     String.Compare(ext, ".dll", StringComparison.OrdinalIgnoreCase) = 0
 
-let IsExe fileName =
+let IsExe (fileName:string) =
     let ext = Path.GetExtension fileName
     String.Compare(ext, ".exe", StringComparison.OrdinalIgnoreCase) = 0
 
@@ -541,7 +541,7 @@ type TcConfig with
                         yield! tcConfig.GetSearchPathsForLibraryFiles()
 
                         if isHashRReference m then
-                            Path.GetDirectoryName(m.FileName)
+                            !! Path.GetDirectoryName(m.FileName)
                     }
 
                 let resolved = TryResolveFileUsingPaths(searchPaths, m, nm)
@@ -989,7 +989,7 @@ type RawFSharpAssemblyDataBackedByFileOnDisk(ilModule: ILModuleDef, ilAssemblyRe
 
             let sigDataReaders =
                 if sigDataReaders.IsEmpty && List.contains ilShortAssemName externalSigAndOptData then
-                    let sigFileName = Path.ChangeExtension(fileName, "sigdata")
+                    let sigFileName = !! Path.ChangeExtension(fileName, "sigdata")
 
                     if not (FileSystem.FileExistsShim sigFileName) then
                         error (Error(FSComp.SR.buildExpectedSigdataFile (FileSystem.GetFullPathShim sigFileName), m))
@@ -1014,7 +1014,7 @@ type RawFSharpAssemblyDataBackedByFileOnDisk(ilModule: ILModuleDef, ilAssemblyRe
             // Look for optimization data in a file
             let optDataReaders =
                 if optDataReaders.IsEmpty && List.contains ilShortAssemName externalSigAndOptData then
-                    let optDataFile = Path.ChangeExtension(fileName, "optdata")
+                    let optDataFile = !! Path.ChangeExtension(fileName, "optdata")
 
                     if not (FileSystem.FileExistsShim optDataFile) then
                         error (
@@ -1464,7 +1464,7 @@ and [<Sealed>] TcImports
         | Tainted.Null -> false, None
         | Tainted.NonNull assembly ->
             let aname = assembly.PUntaint((fun a -> a.GetName()), m)
-            let ilShortAssemName = aname.Name
+            let ilShortAssemName = string aname.Name
 
             match tcImports.FindCcu(ctok, m, ilShortAssemName, lookupOnly = true) with
             | ResolvedCcu ccu ->
@@ -1477,7 +1477,7 @@ and [<Sealed>] TcImports
             | UnresolvedCcu _ ->
                 let g = tcImports.GetTcGlobals()
                 let ilScopeRef = ILScopeRef.Assembly(ILAssemblyRef.FromAssemblyName aname)
-                let fileName = aname.Name + ".dll"
+                let fileName = string aname.Name + ".dll"
 
                 let bytes =
                     assembly
@@ -1860,7 +1860,7 @@ and [<Sealed>] TcImports
                     |> Option.get
                 // MSDN: this method causes the file to be opened and closed, but the assembly is not added to this domain
                 let name = AssemblyName.GetAssemblyName(resolution.resolvedPath)
-                name.Version
+                !! name.Version
 
             // Note, this only captures systemRuntimeContainsTypeRef (which captures tcImportsWeak, using name tcImports)
             let systemRuntimeContainsType =
@@ -1961,7 +1961,7 @@ and [<Sealed>] TcImports
 
             match providers with
             | [] ->
-                let typeName = typeof<TypeProviderAssemblyAttribute>.FullName
+                let typeName = !! typeof<TypeProviderAssemblyAttribute>.FullName
                 warning (Error(FSComp.SR.etHostingAssemblyFoundWithoutHosts (fileNameOfRuntimeAssembly, typeName), m))
             | _ ->
 

src/Compiler/Driver/CompilerOptions.fs

@@ -248,7 +248,7 @@ module ResponseFile =
             let data =
                 seq {
                     while not reader.EndOfStream do
-                        reader.ReadLine()
+                        !! reader.ReadLine()
                 }
                 |> Seq.choose parseLine
                 |> List.ofSeq
@@ -680,8 +680,8 @@ let SetEmbedAllSourceSwitch (tcConfigB: TcConfigBuilder) switch =
     else
         tcConfigB.embedAllSource <- false
 
-let setOutFileName tcConfigB path =
-    let outputDir = Path.GetDirectoryName(path)
+let setOutFileName tcConfigB (path: string) =
+    let outputDir = !! Path.GetDirectoryName(path)
     tcConfigB.outputDir <- Some outputDir
     tcConfigB.outputFile <- Some path
 

src/Compiler/Driver/CreateILModule.fs

@@ -523,7 +523,7 @@ module MainModuleBuilder =
                               $"%d{fileVersionInfo.Major}.%d{fileVersionInfo.Minor}.%d{fileVersionInfo.Build}.%d{fileVersionInfo.Revision}")
                              ("ProductVersion", productVersionString)
                              match tcConfig.outputFile with
-                             | Some f -> ("OriginalFilename", Path.GetFileName f)
+                             | Some f -> ("OriginalFilename", !! Path.GetFileName(f))
                              | None -> ()
                              yield! FindAttribute "Comments" "System.Reflection.AssemblyDescriptionAttribute"
                              yield! FindAttribute "FileDescription" "System.Reflection.AssemblyTitleAttribute"

src/Compiler/Driver/FxResolver.fs

@@ -69,7 +69,7 @@ type internal FxResolver
                 | NonNull message -> lock errorslock (fun () -> errorsList.Add(message))
 
             let psi = ProcessStartInfo()
-            psi.FileName <- pathToExe
+            psi.FileName <- !!pathToExe
 
             if workingDir.IsSome then
                 psi.WorkingDirectory <- workingDir.Value
@@ -91,7 +91,7 @@ type internal FxResolver
                 p.BeginOutputReadLine()
                 p.BeginErrorReadLine()
 
-                if not (p.WaitForExit(timeout)) then
+                if not (p.WaitForExit(timeout: int)) then
                     // Timed out resolving throw a diagnostic.
                     raise (TimeoutException(sprintf "Timeout executing command '%s' '%s'" psi.FileName psi.Arguments))
                 else
@@ -213,7 +213,7 @@ type internal FxResolver
         if String.IsNullOrWhiteSpace fileName then
             getFSharpCompilerLocation ()
         else
-            fileName
+            !!fileName
 
     // Compute the framework implementation directory, either of the selected SDK or the currently running process as a backup
     // F# interactive/reflective scenarios use the implementation directory of the currently running process
@@ -284,7 +284,10 @@ type internal FxResolver
             try
                 let asm = typeof<System.ValueTuple<int, int>>.Assembly
 
-                if asm.FullName.StartsWith("System.ValueTuple", StringComparison.OrdinalIgnoreCase) then
+                if
+                    (!!asm.FullName)
+                        .StartsWith("System.ValueTuple", StringComparison.OrdinalIgnoreCase)
+                then
                     Some asm.Location
                 else
                     let valueTuplePath =
@@ -318,7 +321,7 @@ type internal FxResolver
                     version, ""
 
             match Version.TryParse(ver) with
-            | true, v -> v, suffix
+            | true, v -> !!v, suffix
             | false, _ -> zeroVersion, suffix
 
         let compareVersion (v1: Version * string) (v2: Version * string) =
@@ -371,7 +374,7 @@ type internal FxResolver
                     let di = tryGetVersionedSubDirectory "packs/Microsoft.NETCore.App.Ref" version
 
                     match di with
-                    | Some di -> (Some(di.Name), Some(di.Parent.FullName)), warnings
+                    | Some di -> (Some(di.Name), Some((!!di.Parent).FullName)), warnings
                     | None -> (None, None), warnings
             with e ->
                 let warn =
@@ -495,7 +498,7 @@ type internal FxResolver
                 try
                     if FileSystem.FileExistsShim(reference) then
                         // Reference is a path to a file on disk
-                        Path.GetFileNameWithoutExtension(reference), reference
+                        !! Path.GetFileNameWithoutExtension(reference), reference
                     else
                         // Reference is a SimpleAssembly name
                         reference, frameworkPathFromSimpleName reference
@@ -936,7 +939,7 @@ type internal FxResolver
                                     if useFsiAuxLib then
                                         getFsiLibraryImplementationReference ()
                                 ]
-                                |> List.filter (Path.GetFileNameWithoutExtension >> systemAssemblies.Contains)
+                                |> List.filter (Path.GetFileNameWithoutExtension >> (!!) >> systemAssemblies.Contains)
 
                             sdkReferences, false
                         with e ->

src/Compiler/Driver/GraphChecking/Graph.fs

@@ -84,7 +84,7 @@ module internal Graph =
         |> Seq.iter (fun (KeyValue(file, deps)) -> printfn $"{file} -> {deps |> Array.map nodePrinter |> join}")
 
     let print (graph: Graph<'Node>) : unit =
-        printCustom graph (fun node -> node.ToString())
+        printCustom graph (fun node -> node.ToString() |> string)
 
     let serialiseToMermaid (graph: Graph<FileIndex * string>) =
         let sb = StringBuilder()

src/Compiler/Driver/ParseAndCheckInputs.fs

@@ -352,8 +352,8 @@ let PostParseModuleSpecs
 type ModuleNamesDict = Map<string, Map<string, QualifiedNameOfFile>>
 
 /// Checks if a module name is already given and deduplicates the name if needed.
-let DeduplicateModuleName (moduleNamesDict: ModuleNamesDict) fileName (qualNameOfFile: QualifiedNameOfFile) =
-    let path = Path.GetDirectoryName fileName
+let DeduplicateModuleName (moduleNamesDict: ModuleNamesDict) (fileName: string) (qualNameOfFile: QualifiedNameOfFile) =
+    let path = !! Path.GetDirectoryName(fileName)
 
     let path =
         if FileSystem.IsPathRootedShim path then
@@ -434,7 +434,7 @@ let ParseInput
             "ParseAndCheckFile.parseFile"
             [|
                 Activity.Tags.fileName, fileName
-                Activity.Tags.buildPhase, BuildPhase.Parse.ToString()
+                Activity.Tags.buildPhase, !! BuildPhase.Parse.ToString()
                 Activity.Tags.userOpName, userOpName |> Option.defaultValue ""
             |]
 
@@ -884,7 +884,7 @@ let ProcessMetaCommandsFromInput
 
         match args with
         | [ path ] ->
-            let p = if String.IsNullOrWhiteSpace(path) then "" else path
+            let p = if String.IsNullOrWhiteSpace(path) then "" else !!path
 
             hashReferenceF state (m, p, directive)
 

src/Compiler/Driver/ScriptClosure.fs

@@ -159,7 +159,7 @@ module ScriptPreprocessClosure =
             reduceMemoryUsage
         ) =
 
-        let projectDir = Path.GetDirectoryName fileName
+        let projectDir = !! Path.GetDirectoryName(fileName)
         let isInteractive = (codeContext = CodeContext.CompilationAndEvaluation)
         let isInvalidationSupported = (codeContext = CodeContext.Editing)
 
@@ -460,7 +460,7 @@ module ScriptPreprocessClosure =
 
                         let diagnosticsLogger = CapturingDiagnosticsLogger("FindClosureMetaCommands")
                         use _ = UseDiagnosticsLogger diagnosticsLogger
-                        let pathOfMetaCommandSource = Path.GetDirectoryName fileName
+                        let pathOfMetaCommandSource = !! Path.GetDirectoryName(fileName)
                         let preSources = tcConfig.GetAvailableLoadedSources()
 
                         let tcConfigResult, noWarns =

src/Compiler/Driver/StaticLinking.fs

@@ -158,7 +158,7 @@ let StaticLinkILModules
                     match depILModule.Manifest with
                     | Some m ->
                         for ca in m.CustomAttrs.AsArray() do
-                            if ca.Method.MethodRef.DeclaringTypeRef.FullName = typeof<CompilationMappingAttribute>.FullName then
+                            if ca.Method.MethodRef.DeclaringTypeRef.FullName = !!typeof<CompilationMappingAttribute>.FullName then
                                 ca
                     | _ -> ()
             ]

src/Compiler/Driver/fsc.fs

@@ -371,7 +371,7 @@ module InterfaceFileWriter =
         let writeToSeparateFiles (declaredImpls: CheckedImplFile list) =
             for CheckedImplFile(qualifiedNameOfFile = name) as impl in declaredImpls do
                 let fileName =
-                    Path.ChangeExtension(name.Range.FileName, extensionForFile name.Range.FileName)
+                    !! Path.ChangeExtension(name.Range.FileName, extensionForFile name.Range.FileName)
 
                 printfn "writing impl file to %s" fileName
                 use os = FileSystem.OpenFileForWriteShim(fileName, FileMode.Create).GetWriter()
@@ -392,7 +392,7 @@ module InterfaceFileWriter =
 // 2) If not, but FSharp.Core.dll exists beside the compiler binaries, it will copy it to output directory.
 // 3) If not, it will produce an error.
 let CopyFSharpCore (outFile: string, referencedDlls: AssemblyReference list) =
-    let outDir = Path.GetDirectoryName outFile
+    let outDir = !! Path.GetDirectoryName(outFile)
     let fsharpCoreAssemblyName = GetFSharpCoreLibraryName() + ".dll"
     let fsharpCoreDestinationPath = Path.Combine(outDir, fsharpCoreAssemblyName)
 
@@ -412,7 +412,7 @@ let CopyFSharpCore (outFile: string, referencedDlls: AssemblyReference list) =
     | Some referencedFsharpCoreDll -> copyFileIfDifferent referencedFsharpCoreDll.Text fsharpCoreDestinationPath
     | None ->
         let executionLocation = Assembly.GetExecutingAssembly().Location
-        let compilerLocation = Path.GetDirectoryName executionLocation
+        let compilerLocation = !! Path.GetDirectoryName(executionLocation)
 
         let compilerFsharpCoreDllPath =
             Path.Combine(compilerLocation, fsharpCoreAssemblyName)

src/Compiler/FSharp.Compiler.Service.fsproj

@@ -13,6 +13,15 @@
     <AssemblyName>FSharp.Compiler.Service</AssemblyName>
     <AllowCrossTargeting>true</AllowCrossTargeting>
     <DefineConstants>$(DefineConstants);COMPILER</DefineConstants>
+    <CheckNulls>true</CheckNulls>
+	<!-- Nullness checking againts ns20 base class libraries is very weak, the APIs were not updated with annotations. 
+	     Therefore we want to use the latest BCL APIs from NetCurrent.
+		 We are still not building the actual product in NetCurrent, so for official builds we remain as ns2.0.
+		 For 'BUILDING_USING_DOTNET' builds, we still want latest BCL annotations, so that contributors can get related warnings locally.
+
+     On CI, OSX has problems with Xliff targets for net9, skipping via SKIP_NETCURRENT_FSC_BUILD until resolved ( The target "UpdateXlf" does not exist in the project.)
+	-->
+    <TargetFrameworks Condition=" '$(OfficialBuildId)' == '' AND '$(FSharpNetCoreProductDefaultTargetFramework)' != '' AND '$(Configuration)' != 'Proto' AND '$(SKIP_NETCURRENT_FSC_BUILD)' != 'true' ">$(FSharpNetCoreProductDefaultTargetFramework);$(TargetFrameworks)</TargetFrameworks>
     <DefineConstants Condition="'$(FSHARPCORE_USE_PACKAGE)' == 'true'">$(DefineConstants);FSHARPCORE_USE_PACKAGE</DefineConstants>
     <OtherFlags>$(OtherFlags) --extraoptimizationloops:1</OtherFlags>
     <!-- 1182: Unused variables -->
@@ -87,6 +96,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Include="Utilities\NullnessShims.fs" />
     <EmbeddedText Include="FSComp.txt">
       <Link>FSComp.txt</Link>
     </EmbeddedText>
@@ -96,10 +106,11 @@
     <EmbeddedResource Include="FSStrings.resx">
       <Link>FSStrings.resx</Link>
       <LogicalName>FSStrings.resources</LogicalName>
-    </EmbeddedResource>
-
+    </EmbeddedResource>    
     <Compile Include="Utilities\Activity.fsi" />
     <Compile Include="Utilities\Activity.fs" />
+    <Compile Include="Utilities\illib.fsi" />
+    <Compile Include="Utilities\illib.fs" />
     <Compile Include="Utilities\sformat.fsi" />
     <Compile Include="Utilities\sformat.fs" />
     <Compile Include="Utilities\sr.fsi" />
@@ -112,8 +123,6 @@
     <Compile Include="Utilities\EditDistance.fs" />
     <Compile Include="Utilities\TaggedCollections.fsi" />
     <Compile Include="Utilities\TaggedCollections.fs" />
-    <Compile Include="Utilities\illib.fsi" />
-    <Compile Include="Utilities\illib.fs" />
     <Compile Include="Utilities\Cancellable.fsi" />
     <Compile Include="Utilities\Cancellable.fs" />
     <Compile Include="Utilities\FileSystem.fsi" />

src/Compiler/Facilities/AsyncMemoize.fs

@@ -11,14 +11,15 @@ open FSharp.Compiler
 open FSharp.Compiler.BuildGraph
 open FSharp.Compiler.Diagnostics
 open FSharp.Compiler.DiagnosticsLogger
+open Internal.Utilities.Library
 open System.Runtime.CompilerServices
 
 [<AutoOpen>]
 module internal Utils =
 
     /// Return file name with one directory above it
-    let shortPath path =
-        let dirPath = Path.GetDirectoryName path
+    let shortPath (path: string) =
+        let dirPath = !! Path.GetDirectoryName(path)
 
         let dir =
             dirPath.Split Path.DirectorySeparatorChar
@@ -146,7 +147,12 @@ type internal CachingDiagnosticsLogger(originalLogger: DiagnosticsLogger option)
     member _.CapturedDiagnostics = capturedDiagnostics |> Seq.toList
 
 [<DebuggerDisplay("{DebuggerDisplay}")>]
-type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality>
+type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality
+#if !NO_CHECKNULLS
+    and 'TKey:not null
+    and 'TVersion:not null
+#endif
+    >
     (?keepStrongly, ?keepWeakly, ?name: string, ?cancelDuplicateRunningJobs: bool) =
 
     let name = defaultArg name "N/A"
@@ -287,7 +293,7 @@ type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'T
                             key.Version,
                             key.Label,
                             (Running(
-                                TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously),
+                                TaskCompletionSource<'TValue>(TaskCreationOptions.RunContinuationsAsynchronously),
                                 cts,
                                 computation,
                                 DateTime.Now,
@@ -474,7 +480,7 @@ type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'T
             { new ICacheKey<_, _> with
                 member _.GetKey() = key
                 member _.GetVersion() = Unchecked.defaultof<_>
-                member _.GetLabel() = key.ToString()
+                member _.GetLabel() = match key.ToString() with | null -> "" | s -> s
             }
 
         this.Get(wrappedKey, computation)

src/Compiler/Facilities/AsyncMemoize.fsi

@@ -52,7 +52,12 @@ type internal AsyncLock =
 ///
 /// Strongly holds at most one result per key.
 /// </summary>
-type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality> =
+type internal AsyncMemoize<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality
+#if !NO_CHECKNULLS
+    and 'TKey:not null
+    and 'TVersion:not null
+#endif
+    > =
 
     /// <param name="keepStrongly">Maximum number of strongly held results to keep in the cache</param>
     /// <param name="keepWeakly">Maximum number of weakly held results to keep in the cache</param>

src/Compiler/Facilities/CompilerLocation.fs

@@ -8,6 +8,7 @@ open System.IO
 open System.Reflection
 open System.Runtime.InteropServices
 open Microsoft.FSharp.Core
+open Internal.Utilities.Library
 
 #nowarn "44" // ConfigurationSettings is obsolete but the new stuff is horribly complicated.
 
@@ -23,7 +24,10 @@ module internal FSharpEnvironment =
 
     let FSharpProductName = UtilsStrings.SR.buildProductName (FSharpBannerVersion)
 
-    let versionOf<'t> = typeof<'t>.Assembly.GetName().Version.ToString()
+    let versionOf<'t> : MaybeNull<string> =
+        match typeof<'t>.Assembly.GetName().Version with
+        | null -> null
+        | v -> v.ToString()
 
     let FSharpCoreLibRunningVersion =
         try
@@ -40,8 +44,9 @@ module internal FSharpEnvironment =
     let FSharpBinaryMetadataFormatRevision = "2.0.0.0"
 
     let isRunningOnCoreClr =
-        typeof<obj>.Assembly.FullName
-            .StartsWith("System.Private.CoreLib", StringComparison.InvariantCultureIgnoreCase)
+        match typeof<obj>.Assembly.FullName with
+        | null -> false
+        | name -> name.StartsWith("System.Private.CoreLib", StringComparison.InvariantCultureIgnoreCase)
 
     module Option =
         /// Convert string into Option string where null and String.Empty result in None
@@ -69,7 +74,7 @@ module internal FSharpEnvironment =
         try
             // We let you set FSHARP_COMPILER_BIN. I've rarely seen this used and its not documented in the install instructions.
             match Environment.GetEnvironmentVariable("FSHARP_COMPILER_BIN") with
-            | result when not (String.IsNullOrWhiteSpace result) -> Some result
+            | result when not (String.IsNullOrWhiteSpace result) -> Some !!result
             | _ ->
                 let safeExists f =
                     (try
@@ -83,7 +88,8 @@ module internal FSharpEnvironment =
                 | _ ->
                     let fallback () =
                         let d = Assembly.GetExecutingAssembly()
-                        Some(Path.GetDirectoryName d.Location)
+
+                        Some(!! Path.GetDirectoryName(d.Location))
 
                     match tryCurrentDomain () with
                     | None -> fallback ()
@@ -185,7 +191,7 @@ module internal FSharpEnvironment =
                 | Some(p: string) ->
                     match Path.GetDirectoryName(p) with
                     | s when String.IsNullOrEmpty(s) || Path.GetFileName(p) = "packages" || s = p -> ()
-                    | parentDir -> yield! searchParentDirChain (Some parentDir) assemblyName
+                    | parentDir -> yield! searchParentDirChain (Option.ofObj parentDir) assemblyName
 
                 for p in searchToolPaths path compilerToolPaths do
                     let fileName = Path.Combine(p, assemblyName)
@@ -196,7 +202,9 @@ module internal FSharpEnvironment =
 
         let loadFromParentDirRelativeToRuntimeAssemblyLocation designTimeAssemblyName =
             let runTimeAssemblyPath = Path.GetDirectoryName runTimeAssemblyFileName
-            let paths = searchParentDirChain (Some runTimeAssemblyPath) designTimeAssemblyName
+
+            let paths =
+                searchParentDirChain (Option.ofObj runTimeAssemblyPath) designTimeAssemblyName
 
             paths
             |> Seq.tryHead
@@ -204,7 +212,7 @@ module internal FSharpEnvironment =
                 | Some res -> loadFromLocation res
                 | None ->
                     // The search failed, just load from the first location and report an error
-                    let runTimeAssemblyPath = Path.GetDirectoryName runTimeAssemblyFileName
+                    let runTimeAssemblyPath = !! Path.GetDirectoryName(runTimeAssemblyFileName)
                     loadFromLocation (Path.Combine(runTimeAssemblyPath, designTimeAssemblyName))
 
         if designTimeAssemblyName.EndsWith(".dll", StringComparison.OrdinalIgnoreCase) then
@@ -215,9 +223,9 @@ module internal FSharpEnvironment =
             // design-time DLLs specified using "x.DesignTIme, Version= ..." long assembly names and GAC loads.
             // These kind of design-time assembly specifications are no longer used to our knowledge so that comparison is basically legacy
             // and will always succeed.
-            let name = AssemblyName(Path.GetFileNameWithoutExtension designTimeAssemblyName)
+            let name = AssemblyName(!! Path.GetFileNameWithoutExtension(designTimeAssemblyName))
 
-            if name.Name.Equals(name.FullName, StringComparison.OrdinalIgnoreCase) then
+            if name.FullName.Equals(name.Name, StringComparison.OrdinalIgnoreCase) then
                 let designTimeFileName = designTimeAssemblyName + ".dll"
                 loadFromParentDirRelativeToRuntimeAssemblyLocation designTimeFileName
             else
@@ -237,7 +245,8 @@ module internal FSharpEnvironment =
     let getFSharpCompilerLocationWithDefaultFromType (defaultLocation: Type) =
         let location =
             try
-                Some(Path.GetDirectoryName(defaultLocation.Assembly.Location))
+                let directory = Path.GetDirectoryName(defaultLocation.Assembly.Location)
+                Option.ofObj (directory)
             with _ ->
                 None
 
@@ -266,7 +275,7 @@ module internal FSharpEnvironment =
 
     // Must be alongside the location of FSharp.CompilerService.dll
     let getDefaultFsiLibraryLocation () =
-        Path.Combine(Path.GetDirectoryName(getFSharpCompilerLocation ()), fsiLibraryName + ".dll")
+        Path.Combine(!! Path.GetDirectoryName(getFSharpCompilerLocation ()), fsiLibraryName + ".dll")
 
     let isWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
 
@@ -286,7 +295,7 @@ module internal FSharpEnvironment =
             if String.IsNullOrEmpty(pf) then
                 Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles)
             else
-                pf
+                !!pf
 
         let candidate = Path.Combine(pf, "dotnet", dotnet)
 
@@ -311,20 +320,23 @@ module internal FSharpEnvironment =
         let probePathForDotnetHost () =
             let paths =
                 let p = Environment.GetEnvironmentVariable("PATH")
-                if not (isNull p) then p.Split(Path.PathSeparator) else [||]
+
+                match p with
+                | null -> [||]
+                | p -> p.Split(Path.PathSeparator)
 
             paths |> Array.tryFind (fun f -> fileExists (Path.Combine(f, dotnet)))
 
         match (Environment.GetEnvironmentVariable("DOTNET_HOST_PATH")) with
         // Value set externally
-        | value when not (String.IsNullOrEmpty(value)) && fileExists value -> Some value
+        | NonEmptyString value when fileExists value -> Some value
         | _ ->
             // Probe for netsdk install, dotnet. and dotnet.exe is a constant offset from the location of System.Int32
             let candidate =
                 let assemblyLocation =
                     Path.GetDirectoryName(typeof<Int32>.GetTypeInfo().Assembly.Location)
 
-                Path.GetFullPath(Path.Combine(assemblyLocation, "..", "..", "..", dotnet))
+                Path.GetFullPath(Path.Combine(!!assemblyLocation, "..", "..", "..", dotnet))
 
             if fileExists candidate then
                 Some candidate
@@ -342,12 +354,12 @@ module internal FSharpEnvironment =
         [|
             match getDotnetHostPath (), getDotnetGlobalHostPath () with
             | Some hostPath, Some globalHostPath ->
-                yield Path.GetDirectoryName(hostPath)
+                yield !! Path.GetDirectoryName(hostPath)
 
                 if isDotnetMultilevelLookup && hostPath <> globalHostPath then
-                    yield Path.GetDirectoryName(globalHostPath)
-            | Some hostPath, None -> yield Path.GetDirectoryName(hostPath)
-            | None, Some globalHostPath -> yield Path.GetDirectoryName(globalHostPath)
+                    yield !! Path.GetDirectoryName(globalHostPath)
+            | Some hostPath, None -> yield !! Path.GetDirectoryName(hostPath)
+            | None, Some globalHostPath -> yield !! Path.GetDirectoryName(globalHostPath)
             | None, None -> ()
         |]
 

src/Compiler/Facilities/DiagnosticsLogger.fs

@@ -176,7 +176,7 @@ let rec AttachRange m (exn: exn) =
     else
         match exn with
         // Strip TargetInvocationException wrappers
-        | :? TargetInvocationException -> AttachRange m exn.InnerException
+        | :? TargetInvocationException as e when isNotNull e.InnerException -> AttachRange m !!exn.InnerException
         | UnresolvedReferenceNoRange a -> UnresolvedReferenceError(a, m)
         | UnresolvedPathReferenceNoRange(a, p) -> UnresolvedPathReference(a, p, m)
         | :? NotSupportedException -> exn
@@ -426,7 +426,7 @@ module DiagnosticsLoggerExtensions =
         try
             if not tryAndDetectDev15 then
                 let preserveStackTrace =
-                    typeof<Exception>
+                    !!typeof<Exception>
                         .GetMethod("InternalPreserveStackTrace", BindingFlags.Instance ||| BindingFlags.NonPublic)
 
                 preserveStackTrace.Invoke(exn, null) |> ignore

src/Compiler/Facilities/SimulatedMSBuildReferenceResolver.fs

@@ -54,7 +54,7 @@ let private SimulatedMSBuildResolver =
         let isDesktop = typeof<int>.Assembly.GetName().Name = "mscorlib"
 
         if isDesktop then
-            match System.Runtime.InteropServices.RuntimeEnvironment.GetRuntimeDirectory() with
+            match (System.Runtime.InteropServices.RuntimeEnvironment.GetRuntimeDirectory(): string MaybeNull) with
             | null -> []
             | x -> [ x ]
         else
@@ -82,7 +82,7 @@ let private SimulatedMSBuildResolver =
             if Environment.OSVersion.Platform = PlatformID.Win32NT then
                 let PF =
                     match Environment.GetEnvironmentVariable("ProgramFiles(x86)") with
-                    | null -> Environment.GetEnvironmentVariable("ProgramFiles") // if PFx86 is null, then we are 32-bit and just get PF
+                    | null -> !! Environment.GetEnvironmentVariable("ProgramFiles") // if PFx86 is null, then we are 32-bit and just get PF
                     | s -> s
 
                 PF + @"\Reference Assemblies\Microsoft\Framework\.NETFramework"
@@ -150,14 +150,14 @@ let private SimulatedMSBuildResolver =
                         let fscoreDir0 =
                             let PF =
                                 match Environment.GetEnvironmentVariable("ProgramFiles(x86)") with
-                                | null -> Environment.GetEnvironmentVariable("ProgramFiles")
+                                | null -> !! Environment.GetEnvironmentVariable("ProgramFiles")
                                 | s -> s
 
                             PF
                             + @"\Reference Assemblies\Microsoft\FSharp\.NETFramework\v4.0\"
-                            + n.Version.ToString()
+                            + (!!n.Version).ToString()
 
-                        let trialPath = Path.Combine(fscoreDir0, n.Name + ".dll")
+                        let trialPath = Path.Combine(fscoreDir0, !!n.Name + ".dll")
 
                         if FileSystem.FileExistsShim trialPath then
                             success trialPath
@@ -173,7 +173,7 @@ let private SimulatedMSBuildResolver =
                         r
                     else
                         try
-                            AssemblyName(r).Name + ".dll"
+                            !!AssemblyName(r).Name + ".dll"
                         with _ ->
                             r + ".dll"
 
@@ -198,7 +198,7 @@ let private SimulatedMSBuildResolver =
                         let netFx = System.Runtime.InteropServices.RuntimeEnvironment.GetRuntimeDirectory()
 
                         let gac =
-                            Path.Combine(Path.GetDirectoryName(Path.GetDirectoryName(netFx.TrimEnd('\\'))), "assembly")
+                            Path.Combine(!! Path.GetDirectoryName(Path.GetDirectoryName(netFx.TrimEnd('\\'))), "assembly")
 
                         match n.Version, n.GetPublicKeyToken() with
                         | null, _
@@ -207,7 +207,7 @@ let private SimulatedMSBuildResolver =
                                 [
                                     if FileSystem.DirectoryExistsShim gac then
                                         for gacDir in FileSystem.EnumerateDirectoriesShim gac do
-                                            let assemblyDir = Path.Combine(gacDir, n.Name)
+                                            let assemblyDir = Path.Combine(gacDir, !!n.Name)
 
                                             if FileSystem.DirectoryExistsShim assemblyDir then
                                                 for tdir in FileSystem.EnumerateDirectoriesShim assemblyDir do
@@ -228,7 +228,7 @@ let private SimulatedMSBuildResolver =
                             if FileSystem.DirectoryExistsShim gac then
                                 for gacDir in Directory.EnumerateDirectories gac do
                                     //printfn "searching GAC directory: %s" gacDir
-                                    let assemblyDir = Path.Combine(gacDir, n.Name)
+                                    let assemblyDir = Path.Combine(gacDir, !!n.Name)
 
                                     if FileSystem.DirectoryExistsShim assemblyDir then
                                         //printfn "searching GAC directory: %s" assemblyDir

src/Compiler/Facilities/prim-lexing.fs

@@ -49,7 +49,7 @@ type StringText(str: string) =
             let mutable line = reader.ReadLine()
 
             while not (isNull line) do
-                yield line
+                yield !!line
                 line <- reader.ReadLine()
 
             if str.EndsWith("\n", StringComparison.Ordinal) then
@@ -155,7 +155,9 @@ type StringText(str: string) =
                     sb.Append(lastLine.Substring(0, range.EndColumn)).ToString()
 
         member _.GetChecksum() =
-            str |> Md5Hasher.hashString |> ImmutableArray.Create<byte>
+            str
+            |> Md5Hasher.hashString
+            |> fun byteArray -> ImmutableArray.Create<byte>(byteArray, 0, byteArray.Length)
 
 module SourceText =
 
@@ -190,7 +192,9 @@ module SourceTextNew =
 
             member _.GetChecksum() =
                 // TODO: something better...
-                sourceText.ToString() |> Md5Hasher.hashString |> ImmutableArray.Create<byte>
+                !! sourceText.ToString()
+                |> Md5Hasher.hashString
+                |> fun byteArray -> ImmutableArray.Create<byte>(byteArray, 0, byteArray.Length)
         }
 
 // NOTE: the code in this file is a drop-in replacement runtime for Lexing.fs from the FsLexYacc repository

src/Compiler/Interactive/ControlledExecution.fs

@@ -13,6 +13,7 @@ open System.Reflection
 open System.Threading
 
 open Internal.Utilities.FSharpEnvironment
+open Internal.Utilities.Library
 
 open Unchecked
 
@@ -24,21 +25,20 @@ type internal ControlledExecution(isInteractive: bool) =
     static let ceType: Type option =
         Option.ofObj (Type.GetType("System.Runtime.ControlledExecution, System.Private.CoreLib", false))
 
-    static let threadType: Type option = Option.ofObj (typeof<Threading.Thread>)
+    static let threadType: Type option = typeof<Threading.Thread> |> Option.ofObj
 
     static let ceRun: MethodInfo option =
         match ceType with
         | None -> None
         | Some t ->
-            Option.ofObj (
-                t.GetMethod(
-                    "Run",
-                    BindingFlags.Static ||| BindingFlags.Public,
-                    defaultof<Binder>,
-                    [| typeof<System.Action>; typeof<System.Threading.CancellationToken> |],
-                    [||]
-                )
+            t.GetMethod(
+                "Run",
+                BindingFlags.Static ||| BindingFlags.Public,
+                defaultof<Binder>,
+                [| typeof<System.Action>; typeof<System.Threading.CancellationToken> |],
+                [||]
             )
+            |> Option.ofObj
 
     static let threadResetAbort: MethodInfo option =
         match isRunningOnCoreClr, threadType with
@@ -67,6 +67,8 @@ type internal ControlledExecution(isInteractive: bool) =
 
     static member StripTargetInvocationException(exn: Exception) =
         match exn with
-        | :? TargetInvocationException as e when not (isNull e.InnerException) ->
-            ControlledExecution.StripTargetInvocationException(e.InnerException)
+        | :? TargetInvocationException as e ->
+            match e.InnerException with
+            | null -> exn
+            | innerEx -> ControlledExecution.StripTargetInvocationException(innerEx)
         | _ -> exn

src/Compiler/Interactive/fsi.fs

@@ -374,7 +374,7 @@ type ILMultiInMemoryAssemblyEmitEnv
         let typT = convTypeRef tref
         let tyargs = List.map convTypeAux tspec.GenericArgs
 
-        let res =
+        let res: Type MaybeNull =
             match isNil tyargs, typT.IsGenericType with
             | _, true -> typT.MakeGenericType(List.toArray tyargs)
             | true, false -> typT
@@ -389,7 +389,7 @@ type ILMultiInMemoryAssemblyEmitEnv
 
     and convTypeAux ty =
         match ty with
-        | ILType.Void -> Type.GetType("System.Void")
+        | ILType.Void -> !! Type.GetType("System.Void")
         | ILType.Array(shape, eltType) ->
             let baseT = convTypeAux eltType
 
@@ -397,8 +397,8 @@ type ILMultiInMemoryAssemblyEmitEnv
                 baseT.MakeArrayType()
             else
                 baseT.MakeArrayType shape.Rank
-        | ILType.Value tspec -> convTypeSpec tspec
-        | ILType.Boxed tspec -> convTypeSpec tspec
+        | ILType.Value tspec -> !!(convTypeSpec tspec)
+        | ILType.Boxed tspec -> !!(convTypeSpec tspec)
         | ILType.Ptr eltType ->
             let baseT = convTypeAux eltType
             baseT.MakePointerType()
@@ -436,7 +436,7 @@ type ILMultiInMemoryAssemblyEmitEnv
         let ltref = mkRefForNestedILTypeDef ILScopeRef.Local (enc, tdef)
         let tref = mkRefForNestedILTypeDef ilScopeRef (enc, tdef)
         let key = tref.BasicQualifiedName
-        let typ = asm.GetType(key)
+        let typ = !! asm.GetType(key)
         //printfn "Adding %s --> %s" key typ.FullName
         let rtref = rescopeILTypeRef dynamicCcuScopeRef tref
         typeMap.Add(ltref, (typ, tref))
@@ -511,7 +511,7 @@ type FsiEvaluationSessionHostConfig() =
     abstract FloatingPointFormat: string
 
     /// Called by the evaluation session to ask the host for parameters to format text for output
-    abstract AddedPrinters: Choice<Type * (obj -> string), Type * (obj -> obj)> list
+    abstract AddedPrinters: Choice<Type * (objnull -> string), Type * (objnull -> objnull)> list
 
     /// Called by the evaluation session to ask the host for parameters to format text for output
     abstract ShowDeclarationValues: bool
@@ -588,7 +588,7 @@ type FsiEvaluationSessionHostConfig() =
 type internal FsiValuePrinter(fsi: FsiEvaluationSessionHostConfig, outWriter: TextWriter) =
 
     /// This printer is used by F# Interactive if no other printers apply.
-    let DefaultPrintingIntercept (ienv: IEnvironment) (obj: obj) =
+    let DefaultPrintingIntercept (ienv: IEnvironment) (obj: objnull) =
         match obj with
         | null -> None
         | :? System.Collections.IDictionary as ie ->
@@ -630,10 +630,10 @@ type internal FsiValuePrinter(fsi: FsiEvaluationSessionHostConfig, outWriter: Te
                         match x with
                         | Choice1Of2(aty: Type, printer) ->
                             yield
-                                (fun _ienv (obj: obj) ->
+                                (fun _ienv (obj: objnull) ->
                                     match obj with
                                     | null -> None
-                                    | _ when aty.IsAssignableFrom(obj.GetType()) ->
+                                    | obj when aty.IsAssignableFrom(obj.GetType()) ->
                                         let text = printer obj
 
                                         match box text with
@@ -643,10 +643,10 @@ type internal FsiValuePrinter(fsi: FsiEvaluationSessionHostConfig, outWriter: Te
 
                         | Choice2Of2(aty: Type, converter) ->
                             yield
-                                (fun ienv (obj: obj) ->
+                                (fun ienv (obj: objnull) ->
                                     match obj with
                                     | null -> None
-                                    | _ when aty.IsAssignableFrom(obj.GetType()) ->
+                                    | obj when aty.IsAssignableFrom(obj.GetType()) ->
                                         match converter obj with
                                         | null -> None
                                         | res -> Some(ienv.GetLayout res)
@@ -938,8 +938,8 @@ let internal directoryName (s: string) =
         "."
     else
         match Path.GetDirectoryName s with
-        | null -> if FileSystem.IsPathRootedShim s then s else "."
-        | res -> if String.IsNullOrEmpty(res) then "." else res
+        | Null -> if FileSystem.IsPathRootedShim s then s else "."
+        | NonNull res -> if String.IsNullOrEmpty(res) then "." else res
 
 //----------------------------------------------------------------------------
 // cmd line - state for options
@@ -976,10 +976,11 @@ type internal FsiCommandLineOptions(fsi: FsiEvaluationSessionHostConfig, argv: s
     let executableFileNameWithoutExtension =
         lazy
             let getFsiCommandLine () =
-                let fileNameWithoutExtension path = Path.GetFileNameWithoutExtension(path)
+                let fileNameWithoutExtension (path: string MaybeNull) = Path.GetFileNameWithoutExtension(path)
 
                 let currentProcess = Process.GetCurrentProcess()
-                let processFileName = fileNameWithoutExtension currentProcess.MainModule.FileName
+                let mainModule = currentProcess.MainModule
+                let processFileName = fileNameWithoutExtension (mainModule ^ _.FileName)
 
                 let commandLineExecutableFileName =
                     try
@@ -994,7 +995,7 @@ type internal FsiCommandLineOptions(fsi: FsiEvaluationSessionHostConfig, argv: s
                     | _ -> StringComparison.OrdinalIgnoreCase
 
                 if String.Compare(processFileName, commandLineExecutableFileName, stringComparison) = 0 then
-                    processFileName
+                    !!processFileName
                 else
                     sprintf "%s %s" processFileName commandLineExecutableFileName
 
@@ -1530,7 +1531,7 @@ let ConvReflectionTypeToILTypeRef (reflectionTy: Type) =
     let aref = ILAssemblyRef.FromAssemblyName(reflectionTy.Assembly.GetName())
     let scoref = ILScopeRef.Assembly aref
 
-    let fullName = reflectionTy.FullName
+    let fullName = reflectionTy.FullName |> nullArgCheck "reflectionTy.FullName"
     let index = fullName.IndexOfOrdinal("[")
 
     let fullName =
@@ -1569,10 +1570,10 @@ let rec ConvReflectionTypeToILType (reflectionTy: Type) =
                 && IsCompilerGeneratedName reflectionTy.Name
             then
                 let rec get (typ: Type) =
-                    if FSharp.Reflection.FSharpType.IsFunction typ.BaseType then
-                        get typ.BaseType
-                    else
-                        typ
+                    match typ.BaseType with
+                    | null -> typ
+                    | baseTyp when FSharp.Reflection.FSharpType.IsFunction baseTyp -> get baseTyp
+                    | _ -> typ
 
                 get reflectionTy
             else
@@ -1582,7 +1583,7 @@ let rec ConvReflectionTypeToILType (reflectionTy: Type) =
 
     let elementOrItemTref =
         if reflectionTy.HasElementType then
-            reflectionTy.GetElementType()
+            !! reflectionTy.GetElementType()
         else
             reflectionTy
         |> ConvReflectionTypeToILTypeRef
@@ -1675,7 +1676,7 @@ let internal mkBoundValueTypedImpl tcGlobals m moduleName name ty =
     entity, v, CheckedImplFile.CheckedImplFile(qname, [], mty, contents, false, false, StampMap.Empty, Map.empty)
 
 let scriptingSymbolsPath =
-    let createDirectory path =
+    let createDirectory (path: string) =
         lazy
             try
                 if not (Directory.Exists(path)) then
@@ -1906,7 +1907,7 @@ type internal FsiDynamicCompiler
                     if edef.ArgCount = 0 then
                         yield
                             (fun () ->
-                                let typ = asm.GetType(edef.DeclaringTypeRef.BasicQualifiedName)
+                                let typ = !! asm.GetType(edef.DeclaringTypeRef.BasicQualifiedName)
 
                                 try
                                     ignore (
@@ -1924,8 +1925,8 @@ type internal FsiDynamicCompiler
                                     )
 
                                     None
-                                with :? TargetInvocationException as e ->
-                                    Some e.InnerException)
+                                with :? TargetInvocationException as e when isNotNull e.InnerException ->
+                                    Some !!e.InnerException)
             ]
 
         emEnv.AddModuleDef asm ilScopeRef ilxMainModule
@@ -2418,7 +2419,7 @@ type internal FsiDynamicCompiler
     member _.DynamicAssemblies = dynamicAssemblies.ToArray()
 
     member _.FindDynamicAssembly(name, useFullName: bool) =
-        let getName (assemblyName: AssemblyName) =
+        let getName (assemblyName: AssemblyName) : string MaybeNull =
             if useFullName then
                 assemblyName.FullName
             else
@@ -2847,7 +2848,7 @@ type internal FsiDynamicCompiler
 
                      st),
                  (fun _ _ -> ()))
-                (tcConfigB, input, Path.GetDirectoryName sourceFile, istate))
+                (tcConfigB, input, !! Path.GetDirectoryName(sourceFile), istate))
 
     member fsiDynamicCompiler.EvalSourceFiles(ctok, istate, m, sourceFiles, lexResourceManager, diagnosticsLogger: DiagnosticsLogger) =
         let tcConfig = TcConfig.Create(tcConfigB, validate = false)
@@ -2942,11 +2943,9 @@ type internal FsiDynamicCompiler
             | _ -> None
         | _ -> None
 
-    member _.AddBoundValue(ctok, diagnosticsLogger: DiagnosticsLogger, istate, name: string, value: obj) =
+    member _.AddBoundValue(ctok, diagnosticsLogger: DiagnosticsLogger, istate, name: string, value: objnull) =
         try
-            match value with
-            | null -> nullArg "value"
-            | _ -> ()
+            let value = value |> nullArgCheck (nameof value)
 
             if String.IsNullOrWhiteSpace name then
                 invalidArg "name" "Name cannot be null or white-space."
@@ -3539,7 +3538,7 @@ type FsiStdinLexerProvider
 
                     0
                 | Some(NonNull input) ->
-                    let input = nonNull input + "\n"
+                    let input = input + "\n"
 
                     if input.Length > len then
                         fprintf fsiConsoleOutput.Error "%s" (FSIstrings.SR.fsiLineTooLong ())
@@ -5063,8 +5062,8 @@ module Settings =
         let runSignal = new AutoResetEvent(false)
         let exitSignal = new AutoResetEvent(false)
         let doneSignal = new AutoResetEvent(false)
-        let mutable queue = ([]: (unit -> obj) list)
-        let mutable result = (None: obj option)
+        let mutable queue = ([]: (unit -> objnull) list)
+        let mutable result = (None: objnull option)
 
         let setSignal (signal: AutoResetEvent) =
             while not (signal.Set()) do

src/Compiler/Interactive/fsi.fsi

@@ -8,6 +8,7 @@ open System.Threading
 open FSharp.Compiler.CodeAnalysis
 open FSharp.Compiler.Diagnostics
 open FSharp.Compiler.Symbols
+open Internal.Utilities.Library
 
 /// Represents an evaluated F# value
 [<Class>]
@@ -62,7 +63,7 @@ type public FsiEvaluationSessionHostConfig =
     abstract FloatingPointFormat: string
 
     /// Called by the evaluation session to ask the host for parameters to format text for output
-    abstract AddedPrinters: Choice<Type * (obj -> string), Type * (obj -> obj)> list
+    abstract AddedPrinters: Choice<Type * (objnull -> string), Type * (objnull -> objnull)> list
 
     /// Called by the evaluation session to ask the host for parameters to format text for output
     abstract ShowDeclarationValues: bool
@@ -396,7 +397,7 @@ module Settings =
         /// <summary>Register a print transformer that controls the output of the interactive session.</summary>
         member AddPrintTransformer: ('T -> obj) -> unit
 
-        member internal AddedPrinters: Choice<Type * (obj -> string), Type * (obj -> obj)> list
+        member internal AddedPrinters: Choice<Type * (objnull -> string), Type * (objnull -> objnull)> list
 
         /// <summary>The command line arguments after ignoring the arguments relevant to the interactive
         /// environment and replacing the first argument with the name of the last script file,

src/Compiler/Interactive/fsihelp.fs

@@ -7,6 +7,10 @@ open System.Text
 open System.Reflection
 open FSharp.Compiler.IO
 
+// 3261 Is the nullness warning. I really tried to properly check all accesses, but the chosen xml API has nulles everywhere and is not a good fit for compiler nullness checking.
+// Even basic constructs like `n.Attributes.GetNamedItem("name").Value` have `| null| on every single dot access.
+#nowarn "3261"
+
 module Parser =
 
     open System.Xml

src/Compiler/Legacy/LegacyHostedCompilerForTesting.fs

@@ -187,21 +187,21 @@ type internal FscCompiler(legacyReferenceResolver) =
         let regex =
             Regex(@"^(/|--)test:ErrorRanges$", RegexOptions.Compiled ||| RegexOptions.IgnoreCase)
 
-        fun arg -> regex.IsMatch(arg)
+        fun (arg: string) -> regex.IsMatch(arg)
 
     /// test if --vserrors flag is set
     let vsErrorsArg =
         let regex =
             Regex(@"^(/|--)vserrors$", RegexOptions.Compiled ||| RegexOptions.IgnoreCase)
 
-        fun arg -> regex.IsMatch(arg)
+        fun (arg: string) -> regex.IsMatch(arg)
 
     /// test if an arg is a path to fsc.exe
     let fscExeArg =
         let regex =
             Regex(@"fsc(\.exe)?$", RegexOptions.Compiled ||| RegexOptions.IgnoreCase)
 
-        fun arg -> regex.IsMatch(arg)
+        fun (arg: string) -> regex.IsMatch(arg)
 
     /// do compilation as if args was argv to fsc.exe
     member _.Compile(args: string[]) =
@@ -209,7 +209,7 @@ type internal FscCompiler(legacyReferenceResolver) =
         // compensate for this in case caller didn't know
         let args =
             match box args with
-            | Null -> [| "fsc" |]
+            | null -> [| "fsc" |]
             | _ ->
                 match args with
                 | [||] -> [| "fsc" |]

src/Compiler/Optimize/LowerStateMachines.fs

@@ -448,7 +448,7 @@ type LowerStateMachine(g: TcGlobals) =
         let res = 
             match expr with 
             | ResumableCodeInvoke g (_, _, _, m, _) ->
-                Result.Error (FSComp.SR.reprResumableCodeInvokeNotReduced(m.ToString()))
+                Result.Error (FSComp.SR.reprResumableCodeInvokeNotReduced(!!m.ToString()))
 
             // Eliminate 'if __useResumableCode ...' within.  
             | IfUseResumableStateMachinesExpr g (thenExpr, _) -> 

src/Compiler/Service/FSharpCheckerResults.fs

@@ -203,7 +203,10 @@ module internal FSharpCheckerResultsSettings =
     // Look for DLLs in the location of the service DLL first.
     let defaultFSharpBinariesDir =
         FSharpEnvironment
-            .BinFolderOfDefaultFSharpCompiler(Some(Path.GetDirectoryName(typeof<IncrementalBuilder>.Assembly.Location)))
+            .BinFolderOfDefaultFSharpCompiler(
+                Path.GetDirectoryName(typeof<IncrementalBuilder>.Assembly.Location)
+                |> Option.ofObj
+            )
             .Value
 
 [<Sealed>]
@@ -987,7 +990,7 @@ type internal TypeCheckInfo
         if String.IsNullOrWhiteSpace name then
             None
         else
-            let name = String.lowerCaseFirstChar name
+            let name = String.lowerCaseFirstChar !!name
 
             let unused =
                 sResolutions.CapturedNameResolutions
@@ -3019,7 +3022,7 @@ module internal ParseAndCheckFile =
     let parseFile
         (
             sourceText: ISourceText,
-            fileName,
+            fileName: string,
             options: FSharpParsingOptions,
             userOpName: string,
             suggestNamesForErrors: bool,
@@ -3074,7 +3077,7 @@ module internal ParseAndCheckFile =
         (
             tcConfig,
             parsedMainInput,
-            mainInputFileName,
+            mainInputFileName: string,
             loadClosure: LoadClosure option,
             tcImports: TcImports,
             backgroundDiagnostics
@@ -3166,7 +3169,7 @@ module internal ParseAndCheckFile =
             ApplyMetaCommandsFromInputToTcConfig(
                 tcConfig,
                 parsedMainInput,
-                Path.GetDirectoryName mainInputFileName,
+                !! Path.GetDirectoryName(mainInputFileName),
                 tcImports.DependencyProvider
             )
             |> ignore
@@ -3217,7 +3220,7 @@ module internal ParseAndCheckFile =
 
             // Apply nowarns to tcConfig (may generate errors, so ensure diagnosticsLogger is installed)
             let tcConfig =
-                ApplyNoWarnsToTcConfig(tcConfig, parsedMainInput, Path.GetDirectoryName mainInputFileName)
+                ApplyNoWarnsToTcConfig(tcConfig, parsedMainInput, !! Path.GetDirectoryName(mainInputFileName))
 
             // update the error handler with the modified tcConfig
             errHandler.DiagnosticOptions <- tcConfig.diagnosticsOptions

src/Compiler/Service/FSharpParseFileResults.fs

@@ -14,7 +14,7 @@ open FSharp.Compiler.Text
 open FSharp.Compiler.Text.Range
 
 module SourceFileImpl =
-    let IsSignatureFile file =
+    let IsSignatureFile (file: string) =
         let ext = Path.GetExtension file
         0 = String.Compare(".fsi", ext, StringComparison.OrdinalIgnoreCase)
 

src/Compiler/Service/FSharpProjectSnapshot.fs

@@ -7,6 +7,7 @@ open System.Collections.Generic
 open System.IO
 open System.Reflection
 open FSharp.Compiler.IO
+open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
 open FSharp.Core.Printf
 open FSharp.Compiler.Text
@@ -419,7 +420,7 @@ and internal ProjectCore
                  member _.GetVersion() = fullHashString.Value
              })
 
-    member val ProjectDirectory = Path.GetDirectoryName(ProjectFileName)
+    member val ProjectDirectory = !! Path.GetDirectoryName(ProjectFileName)
     member _.OutputFileName = outputFileName.Value
     member _.Identifier: ProjectIdentifier = key.Value
     member _.Version = fullHash.Value

src/Compiler/Service/IncrementalBuild.fs

@@ -142,7 +142,7 @@ module IncrementalBuildSyntaxTree =
                     Activity.start "IncrementalBuildSyntaxTree.parse"
                         [|
                             Activity.Tags.fileName, fileName
-                            Activity.Tags.buildPhase, BuildPhase.Parse.ToString()
+                            Activity.Tags.buildPhase, !! BuildPhase.Parse.ToString()
                         |]
 
                 try
@@ -264,7 +264,7 @@ type BoundModel private (
 
             beforeFileChecked.Trigger fileName
                     
-            ApplyMetaCommandsFromInputToTcConfig (tcConfig, input, Path.GetDirectoryName fileName, tcImports.DependencyProvider) |> ignore
+            ApplyMetaCommandsFromInputToTcConfig (tcConfig, input, !! Path.GetDirectoryName(fileName), tcImports.DependencyProvider) |> ignore
             let sink = TcResultsSinkImpl(tcGlobals)
             let hadParseErrors = not (Array.isEmpty parseErrors)
             let input, moduleNamesDict = DeduplicateParsedInputModuleName prevTcInfo.moduleNamesDict input
@@ -805,7 +805,7 @@ module IncrementalBuilderHelpers =
                         let hasTypeProviderAssemblyAttrib =
                             topAttrs.assemblyAttrs |> List.exists (fun (Attrib(tcref, _, _, _, _, _, _)) ->
                                 let nm = tcref.CompiledRepresentationForNamedType.BasicQualifiedName
-                                nm = typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName)
+                                nm = !! typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName)
 
                         if tcState.CreatesGeneratedProvidedTypes || hasTypeProviderAssemblyAttrib then
                             ProjectAssemblyDataResult.Unavailable true
@@ -1455,7 +1455,7 @@ type IncrementalBuilder(initialState: IncrementalBuilderInitialState, state: Inc
                     { new IXmlDocumentationInfoLoader with
                         /// Try to load xml documentation associated with an assembly by the same file path with the extension ".xml".
                         member _.TryLoad(assemblyFileName) =
-                            let xmlFileName = Path.ChangeExtension(assemblyFileName, ".xml")
+                            let xmlFileName = !! Path.ChangeExtension(assemblyFileName, ".xml")
 
                             // REVIEW: File IO - Will eventually need to change this to use a file system interface of some sort.
                             XmlDocumentationInfo.TryCreateFromFile(xmlFileName)

src/Compiler/Service/ItemKey.fs

@@ -7,6 +7,7 @@ open System.IO
 open System.IO.MemoryMappedFiles
 open System.Reflection.Metadata
 open System.Runtime.InteropServices
+open Internal.Utilities.Library
 open FSharp.NativeInterop
 open FSharp.Compiler.AbstractIL.IL
 open FSharp.Compiler.Infos
@@ -436,7 +437,7 @@ and [<Sealed>] ItemKeyStoreBuilder(tcGlobals: TcGlobals) =
         writeString ItemKeyTags.itemActivePattern
 
         match apInfo.ActiveTagsWithRanges with
-        | (_, m) :: _ -> m.FileName |> Path.GetFileNameWithoutExtension |> writeString
+        | (_, m) :: _ -> m.FileName |> Path.GetFileNameWithoutExtension |> (!!) |> writeString
         | _ -> ()
 
         for tag in apInfo.ActiveTags do

src/Compiler/Service/ServiceDeclarationLists.fs

@@ -121,7 +121,7 @@ module DeclarationListHelpers =
   
         { new IPartialEqualityComparer<CompletionItem> with
             member x.InEqualityRelation item = itemComparer.InEqualityRelation item.Item
-            member x.Equals(item1, item2) = itemComparer.Equals(item1.Item, item2.Item)
+            member x.Equals(item1, item2) = nullSafeEquality item1 item2 (fun item1 item2 -> itemComparer.Equals(item1.Item, item2.Item))
             member x.GetHashCode item = itemComparer.GetHashCode(item.Item) }
 
     /// Remove all duplicate items
@@ -138,7 +138,7 @@ module DeclarationListHelpers =
         modrefs |> IPartialEqualityComparer.partialDistinctBy 
                       { new IPartialEqualityComparer<ModuleOrNamespaceRef> with
                           member x.InEqualityRelation _ = true
-                          member x.Equals(item1, item2) = (fullDisplayTextOfModRef item1 = fullDisplayTextOfModRef item2)
+                          member x.Equals(item1, item2) = nullSafeEquality item1 item2 (fun item1 item2 -> fullDisplayTextOfModRef item1 = fullDisplayTextOfModRef item2)
                           member x.GetHashCode item = hash item.Stamp  }
 
     let OutputFullName displayFullName ppF fnF r = 
@@ -671,7 +671,7 @@ module internal DescriptionListsImpl =
                 |> Array.map (fun sp -> 
                     let ty = Import.ImportProvidedType amap m (sp.PApply((fun x -> x.ParameterType), m))
                     let spKind = NicePrint.prettyLayoutOfType denv ty
-                    let spName = sp.PUntaint((fun sp -> nonNull sp.Name), m)
+                    let spName = sp.PUntaint((fun sp -> sp.Name), m)
                     let spOpt = sp.PUntaint((fun sp -> sp.IsOptional), m)
                     let display = (if spOpt then SepL.questionMark else emptyL) ^^ wordL (tagParameter spName) ^^ RightL.colon ^^ spKind
                     let display = toArray display

src/Compiler/Service/ServiceInterfaceStubGenerator.fs

@@ -41,7 +41,7 @@ module internal CodeGenerationUtils =
         member _.Unindent i =
             indentWriter.Indent <- max 0 (indentWriter.Indent - i)
 
-        member _.Dump() = indentWriter.InnerWriter.ToString()
+        member _.Dump() = !! indentWriter.InnerWriter.ToString()
 
         interface IDisposable with
             member _.Dispose() =

src/Compiler/Service/ServiceParsedInputOps.fs

@@ -17,7 +17,7 @@ open FSharp.Compiler.Text.Position
 open FSharp.Compiler.Text.Range
 
 module SourceFileImpl =
-    let IsSignatureFile file =
+    let IsSignatureFile (file: string) =
         let ext = Path.GetExtension file
         0 = String.Compare(".fsi", ext, StringComparison.OrdinalIgnoreCase)
 

src/Compiler/Service/SynExpr.fs

@@ -519,7 +519,7 @@ module SynExpr =
         if startLine = endLine then
             range.StartColumn <= outerOffsidesColumn
         else
-            let rec loop offsides lineNo startCol =
+            let rec loop offsides lineNo (startCol: int) =
                 if lineNo <= endLine then
                     let line = getSourceLineStr lineNo
 

src/Compiler/Service/TransparentCompiler.fs

@@ -808,7 +808,7 @@ type internal TransparentCompiler
                 { new IXmlDocumentationInfoLoader with
                     /// Try to load xml documentation associated with an assembly by the same file path with the extension ".xml".
                     member _.TryLoad(assemblyFileName) =
-                        let xmlFileName = Path.ChangeExtension(assemblyFileName, ".xml")
+                        let xmlFileName = !! Path.ChangeExtension(assemblyFileName, ".xml")
 
                         // REVIEW: File IO - Will eventually need to change this to use a file system interface of some sort.
                         XmlDocumentationInfo.TryCreateFromFile(xmlFileName)
@@ -834,7 +834,7 @@ type internal TransparentCompiler
                     Activity.start
                         "ComputeBootstrapInfoStatic"
                         [|
-                            Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName
+                            Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName |> (!!)
                             "references", projectSnapshot.ReferencedProjects.Length.ToString()
                         |]
 
@@ -988,7 +988,11 @@ type internal TransparentCompiler
             projectSnapshot.NoFileVersionsKey,
             async {
                 use _ =
-                    Activity.start "ComputeBootstrapInfo" [| Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName |]
+                    Activity.start
+                        "ComputeBootstrapInfo"
+                        [|
+                            Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName |> (!!)
+                        |]
 
                 // Trap and report diagnostics from creation.
                 let delayedLogger = CapturingDiagnosticsLogger("IncrementalBuilderCreation")
@@ -1156,8 +1160,9 @@ type internal TransparentCompiler
 
             //Trace.TraceInformation("\n" + debugGraph)
 
-            if Activity.Current <> null then
-                Activity.Current.AddTag("graph", debugGraph) |> ignore
+            match Activity.Current with
+            | Null -> ()
+            | NonNull a -> a.AddTag("graph", debugGraph) |> ignore
 
             return nodeGraph, graph
         }
@@ -1261,7 +1266,7 @@ type internal TransparentCompiler
                     Activity.start
                         "ComputeTcIntermediate"
                         [|
-                            Activity.Tags.fileName, fileName |> Path.GetFileName
+                            Activity.Tags.fileName, fileName |> Path.GetFileName |> (!!)
                             "key", key.GetLabel()
                             "version", "-" // key.GetVersion()
                         |]
@@ -1289,7 +1294,7 @@ type internal TransparentCompiler
 
                 // Apply nowarns to tcConfig (may generate errors, so ensure diagnosticsLogger is installed)
                 let tcConfig =
-                    ApplyNoWarnsToTcConfig(tcConfig, parsedMainInput, Path.GetDirectoryName mainInputFileName)
+                    ApplyNoWarnsToTcConfig(tcConfig, parsedMainInput, !! Path.GetDirectoryName(mainInputFileName))
 
                 let diagnosticsLogger = errHandler.DiagnosticsLogger
 
@@ -1300,7 +1305,7 @@ type internal TransparentCompiler
 
                 //beforeFileChecked.Trigger fileName
 
-                ApplyMetaCommandsFromInputToTcConfig(tcConfig, input, Path.GetDirectoryName fileName, tcImports.DependencyProvider)
+                ApplyMetaCommandsFromInputToTcConfig(tcConfig, input, Path.GetDirectoryName fileName |> (!!), tcImports.DependencyProvider)
                 |> ignore
 
                 let sink = TcResultsSinkImpl(tcGlobals, file.SourceText)
@@ -1473,7 +1478,7 @@ type internal TransparentCompiler
                 let file = projectSnapshot.SourceFiles |> List.last
 
                 use _ =
-                    Activity.start "ComputeTcLastFile" [| Activity.Tags.fileName, file.FileName |> Path.GetFileName |]
+                    Activity.start "ComputeTcLastFile" [| Activity.Tags.fileName, file.FileName |> Path.GetFileName |> (!!) |]
 
                 let! projectSnapshot = parseSourceFiles projectSnapshot bootstrapInfo.TcConfig
 
@@ -1527,7 +1532,7 @@ type internal TransparentCompiler
             projectSnapshot.FileKeyWithExtraFileSnapshotVersion fileName,
             async {
                 use _ =
-                    Activity.start "ComputeParseAndCheckFileInProject" [| Activity.Tags.fileName, fileName |> Path.GetFileName |]
+                    Activity.start "ComputeParseAndCheckFileInProject" [| Activity.Tags.fileName, fileName |> Path.GetFileName |> (!!) |]
 
                 match! ComputeBootstrapInfo projectSnapshot with
                 | None, creationDiags -> return emptyParseResult fileName creationDiags, FSharpCheckFileAnswer.Aborted
@@ -1559,7 +1564,7 @@ type internal TransparentCompiler
 
                     // Apply nowarns to tcConfig (may generate errors, so ensure diagnosticsLogger is installed)
                     let tcConfig =
-                        ApplyNoWarnsToTcConfig(bootstrapInfo.TcConfig, parseResults.ParseTree, Path.GetDirectoryName fileName)
+                        ApplyNoWarnsToTcConfig(bootstrapInfo.TcConfig, parseResults.ParseTree, Path.GetDirectoryName fileName |> (!!))
 
                     let diagnosticsOptions = tcConfig.diagnosticsOptions
 
@@ -1641,7 +1646,9 @@ type internal TransparentCompiler
                 use _ =
                     Activity.start
                         "ComputeParseAndCheckAllFilesInProject"
-                        [| Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName |]
+                        [|
+                            Activity.Tags.project, projectSnapshot.ProjectFileName |> Path.GetFileName |> (!!)
+                        |]
 
                 let! projectSnapshot = parseSourceFiles projectSnapshot bootstrapInfo.TcConfig
 
@@ -1746,7 +1753,7 @@ type internal TransparentCompiler
                             |> List.exists (fun (Attrib(tcref, _, _, _, _, _, _)) ->
                                 let nm = tcref.CompiledRepresentationForNamedType.BasicQualifiedName
 
-                                nm = typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName)
+                                nm = !!typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName)
 
                         if tcState.CreatesGeneratedProvidedTypes || hasTypeProviderAssemblyAttrib then
                             ProjectAssemblyDataResult.Unavailable true
@@ -1912,7 +1919,7 @@ type internal TransparentCompiler
             projectSnapshot.FileKey fileName,
             async {
                 use _ =
-                    Activity.start "ComputeSemanticClassification" [| Activity.Tags.fileName, fileName |> Path.GetFileName |]
+                    Activity.start "ComputeSemanticClassification" [| Activity.Tags.fileName, fileName |> Path.GetFileName |> (!!) |]
 
                 let! sinkOpt = tryGetSink fileName projectSnapshot
 
@@ -1942,7 +1949,7 @@ type internal TransparentCompiler
             projectSnapshot.FileKey fileName,
             async {
                 use _ =
-                    Activity.start "ComputeItemKeyStore" [| Activity.Tags.fileName, fileName |> Path.GetFileName |]
+                    Activity.start "ComputeItemKeyStore" [| Activity.Tags.fileName, fileName |> Path.GetFileName |> (!!) |]
 
                 let! sinkOpt = tryGetSink fileName projectSnapshot
 

src/Compiler/Service/service.fs

@@ -751,14 +751,14 @@ type CompilerEnvironment() =
     static member IsScriptFile(fileName: string) = ParseAndCheckInputs.IsScript fileName
 
     /// Whether or not this file is compilable
-    static member IsCompilable file =
+    static member IsCompilable(file: string) =
         let ext = Path.GetExtension file
 
         compilableExtensions
         |> List.exists (fun e -> 0 = String.Compare(e, ext, StringComparison.OrdinalIgnoreCase))
 
     /// Whether or not this file should be a single-file project
-    static member MustBeSingleFileProject file =
+    static member MustBeSingleFileProject(file: string) =
         let ext = Path.GetExtension file
 
         singleFileProjectExtensions

src/Compiler/Symbols/SymbolHelpers.fs

@@ -254,7 +254,7 @@ module internal SymbolHelpers =
         | FSharpXmlDoc.None
         | FSharpXmlDoc.FromXmlText _ -> xmlDoc
         | FSharpXmlDoc.FromXmlFile(dllName, xmlSig) ->
-            TryFindXmlDocByAssemblyNameAndSig infoReader (Path.GetFileNameWithoutExtension dllName) xmlSig
+            TryFindXmlDocByAssemblyNameAndSig infoReader (!!Path.GetFileNameWithoutExtension(dllName)) xmlSig
             |> Option.map FSharpXmlDoc.FromXmlText
             |> Option.defaultValue xmlDoc
 
@@ -406,66 +406,72 @@ module internal SymbolHelpers =
               //| _ -> false
               
           member x.Equals(item1, item2) = 
-            // This may explore assemblies that are not in the reference set.
-            // In this case just bail out and assume items are not equal
-            protectAssemblyExploration false (fun () -> 
-              let equalHeadTypes(ty1, ty2) =
-                  match tryTcrefOfAppTy g ty1 with
-                  | ValueSome tcref1 ->
-                    match tryTcrefOfAppTy g ty2 with
-                    | ValueSome tcref2 -> tyconRefEq g tcref1 tcref2
-                    | _ -> typeEquiv g ty1 ty2
-                  | _ -> typeEquiv g ty1 ty2
+#if !NO_CHECKNULLS
+            match item1,item2 with
+            | null,null -> true
+            | null,_ | _,null -> false
+            | item1,item2 ->
+#endif
+                // This may explore assemblies that are not in the reference set.
+                // In this case just bail out and assume items are not equal
+                protectAssemblyExploration false (fun () -> 
+                  let equalHeadTypes(ty1, ty2) =
+                      match tryTcrefOfAppTy g ty1 with
+                      | ValueSome tcref1 ->
+                        match tryTcrefOfAppTy g ty2 with
+                        | ValueSome tcref2 -> tyconRefEq g tcref1 tcref2
+                        | _ -> typeEquiv g ty1 ty2
+                      | _ -> typeEquiv g ty1 ty2
 
-              ItemsAreEffectivelyEqual g item1 item2 || 
+                  ItemsAreEffectivelyEqual g item1 item2 || 
 
-              // Much of this logic is already covered by 'ItemsAreEffectivelyEqual'
-              match item1, item2 with 
-              | Item.DelegateCtor ty1, Item.DelegateCtor ty2 -> equalHeadTypes(ty1, ty2)
-              | Item.Types(dn1, ty1 :: _), Item.Types(dn2, ty2 :: _) ->
-                  // Bug 4403: We need to compare names as well, because 'int' and 'Int32' are physically the same type, but we want to show both
-                  dn1 = dn2 && equalHeadTypes(ty1, ty2) 
+                  // Much of this logic is already covered by 'ItemsAreEffectivelyEqual'
+                  match item1, item2 with 
+                  | Item.DelegateCtor ty1, Item.DelegateCtor ty2 -> equalHeadTypes(ty1, ty2)
+                  | Item.Types(dn1, ty1 :: _), Item.Types(dn2, ty2 :: _) ->
+                      // Bug 4403: We need to compare names as well, because 'int' and 'Int32' are physically the same type, but we want to show both
+                      dn1 = dn2 && equalHeadTypes(ty1, ty2) 
               
-              // Prefer a type to a DefaultStructCtor, a DelegateCtor and a FakeInterfaceCtor 
-              | ItemWhereTypIsPreferred ty1, ItemWhereTypIsPreferred ty2 -> equalHeadTypes(ty1, ty2) 
+                  // Prefer a type to a DefaultStructCtor, a DelegateCtor and a FakeInterfaceCtor 
+                  | ItemWhereTypIsPreferred ty1, ItemWhereTypIsPreferred ty2 -> equalHeadTypes(ty1, ty2) 
 
-              | Item.ExnCase tcref1, Item.ExnCase tcref2 -> tyconRefEq g tcref1 tcref2
-              | Item.ILField(fld1), Item.ILField(fld2) ->
-                  ILFieldInfo.ILFieldInfosUseIdenticalDefinitions fld1 fld2
-              | Item.CustomOperation (_, _, Some minfo1), Item.CustomOperation (_, _, Some minfo2) -> 
-                    MethInfo.MethInfosUseIdenticalDefinitions minfo1 minfo2
-              | Item.TypeVar (nm1, tp1), Item.TypeVar (nm2, tp2) -> 
-                    (nm1 = nm2) && typarRefEq tp1 tp2
-              | Item.ModuleOrNamespaces(modref1 :: _), Item.ModuleOrNamespaces(modref2 :: _) -> fullDisplayTextOfModRef modref1 = fullDisplayTextOfModRef modref2
-              | Item.SetterArg(id1, _), Item.SetterArg(id2, _) -> Range.equals id1.idRange id2.idRange && id1.idText = id2.idText
-              | Item.MethodGroup(_, meths1, _), Item.MethodGroup(_, meths2, _) -> 
-                  Seq.zip meths1 meths2 |> Seq.forall (fun (minfo1, minfo2) ->
-                    MethInfo.MethInfosUseIdenticalDefinitions minfo1 minfo2)
-              | (Item.Value vref1 | Item.CustomBuilder (_, vref1)), (Item.Value vref2 | Item.CustomBuilder (_, vref2)) -> 
-                  valRefEq g vref1 vref2
-              | Item.ActivePatternCase(APElemRef(_apinfo1, vref1, idx1, _)), Item.ActivePatternCase(APElemRef(_apinfo2, vref2, idx2, _)) ->
-                  idx1 = idx2 && valRefEq g vref1 vref2
-              | Item.UnionCase(UnionCaseInfo(_, ur1), _), Item.UnionCase(UnionCaseInfo(_, ur2), _) -> 
-                  g.unionCaseRefEq ur1 ur2
-              | Item.RecdField(RecdFieldInfo(_, RecdFieldRef(tcref1, n1))), Item.RecdField(RecdFieldInfo(_, RecdFieldRef(tcref2, n2))) -> 
-                  (tyconRefEq g tcref1 tcref2) && (n1 = n2) // there is no direct function as in the previous case
-              | Item.Property(info = pi1s), Item.Property(info = pi2s) -> 
-                  (pi1s, pi2s) ||> List.forall2 PropInfo.PropInfosUseIdenticalDefinitions
-              | Item.Event evt1, Item.Event evt2 -> 
-                  EventInfo.EventInfosUseIdenticalDefinitions evt1 evt2
-              | Item.AnonRecdField(anon1, _, i1, _), Item.AnonRecdField(anon2, _, i2, _) ->
-                 anonInfoEquiv anon1 anon2 && i1 = i2
-              | Item.Trait traitInfo1, Item.Trait traitInfo2 ->
-                 (traitInfo1.MemberLogicalName = traitInfo2.MemberLogicalName)
-              | Item.CtorGroup(_, meths1), Item.CtorGroup(_, meths2) ->
-                  (meths1, meths2)
-                  ||> List.forall2 MethInfo.MethInfosUseIdenticalDefinitions
-              | Item.UnqualifiedType tcrefs1, Item.UnqualifiedType tcrefs2 ->
-                  (tcrefs1, tcrefs2)
-                  ||> List.forall2 (fun tcref1 tcref2 -> tyconRefEq g tcref1 tcref2)
-              | Item.Types(_, [AbbrevOrAppTy(tcref1, _)]), Item.UnqualifiedType([tcref2]) -> tyconRefEq g tcref1 tcref2
-              | Item.UnqualifiedType([tcref1]), Item.Types(_, [AbbrevOrAppTy(tcref2, _)]) -> tyconRefEq g tcref1 tcref2
-              | _ -> false)
+                  | Item.ExnCase tcref1, Item.ExnCase tcref2 -> tyconRefEq g tcref1 tcref2
+                  | Item.ILField(fld1), Item.ILField(fld2) ->
+                      ILFieldInfo.ILFieldInfosUseIdenticalDefinitions fld1 fld2
+                  | Item.CustomOperation (_, _, Some minfo1), Item.CustomOperation (_, _, Some minfo2) -> 
+                        MethInfo.MethInfosUseIdenticalDefinitions minfo1 minfo2
+                  | Item.TypeVar (nm1, tp1), Item.TypeVar (nm2, tp2) -> 
+                        (nm1 = nm2) && typarRefEq tp1 tp2
+                  | Item.ModuleOrNamespaces(modref1 :: _), Item.ModuleOrNamespaces(modref2 :: _) -> fullDisplayTextOfModRef modref1 = fullDisplayTextOfModRef modref2
+                  | Item.SetterArg(id1, _), Item.SetterArg(id2, _) -> Range.equals id1.idRange id2.idRange && id1.idText = id2.idText
+                  | Item.MethodGroup(_, meths1, _), Item.MethodGroup(_, meths2, _) -> 
+                      Seq.zip meths1 meths2 |> Seq.forall (fun (minfo1, minfo2) ->
+                        MethInfo.MethInfosUseIdenticalDefinitions minfo1 minfo2)
+                  | (Item.Value vref1 | Item.CustomBuilder (_, vref1)), (Item.Value vref2 | Item.CustomBuilder (_, vref2)) -> 
+                      valRefEq g vref1 vref2
+                  | Item.ActivePatternCase(APElemRef(_apinfo1, vref1, idx1, _)), Item.ActivePatternCase(APElemRef(_apinfo2, vref2, idx2, _)) ->
+                      idx1 = idx2 && valRefEq g vref1 vref2
+                  | Item.UnionCase(UnionCaseInfo(_, ur1), _), Item.UnionCase(UnionCaseInfo(_, ur2), _) -> 
+                      g.unionCaseRefEq ur1 ur2
+                  | Item.RecdField(RecdFieldInfo(_, RecdFieldRef(tcref1, n1))), Item.RecdField(RecdFieldInfo(_, RecdFieldRef(tcref2, n2))) -> 
+                      (tyconRefEq g tcref1 tcref2) && (n1 = n2) // there is no direct function as in the previous case
+                  | Item.Property(info = pi1s), Item.Property(info = pi2s) -> 
+                      (pi1s, pi2s) ||> List.forall2 PropInfo.PropInfosUseIdenticalDefinitions
+                  | Item.Event evt1, Item.Event evt2 -> 
+                      EventInfo.EventInfosUseIdenticalDefinitions evt1 evt2
+                  | Item.AnonRecdField(anon1, _, i1, _), Item.AnonRecdField(anon2, _, i2, _) ->
+                     anonInfoEquiv anon1 anon2 && i1 = i2
+                  | Item.Trait traitInfo1, Item.Trait traitInfo2 ->
+                     (traitInfo1.MemberLogicalName = traitInfo2.MemberLogicalName)
+                  | Item.CtorGroup(_, meths1), Item.CtorGroup(_, meths2) ->
+                      (meths1, meths2)
+                      ||> List.forall2 MethInfo.MethInfosUseIdenticalDefinitions
+                  | Item.UnqualifiedType tcrefs1, Item.UnqualifiedType tcrefs2 ->
+                      (tcrefs1, tcrefs2)
+                      ||> List.forall2 (fun tcref1 tcref2 -> tyconRefEq g tcref1 tcref2)
+                  | Item.Types(_, [AbbrevOrAppTy(tcref1, _)]), Item.UnqualifiedType([tcref2]) -> tyconRefEq g tcref1 tcref2
+                  | Item.UnqualifiedType([tcref1]), Item.Types(_, [AbbrevOrAppTy(tcref2, _)]) -> tyconRefEq g tcref1 tcref2
+                  | _ -> false)
               
           member x.GetHashCode item =
             // This may explore assemblies that are not in the reference set.

src/Compiler/SyntaxTree/LexHelpers.fs

@@ -477,13 +477,14 @@ module Keywords =
                         fileName
                         |> FileSystem.GetFullPathShim (* asserts that path is already absolute *)
                         |> System.IO.Path.GetDirectoryName
+                        |> (!!)
 
                 if String.IsNullOrEmpty dirname then
                     dirname
                 else
                     PathMap.applyDir args.pathMap dirname
                 |> fun dir -> KEYWORD_STRING(s, dir)
-            | "__SOURCE_FILE__" -> KEYWORD_STRING(s, System.IO.Path.GetFileName(FileIndex.fileOfFileIndex lexbuf.StartPos.FileIndex))
+            | "__SOURCE_FILE__" -> KEYWORD_STRING(s, !! System.IO.Path.GetFileName(FileIndex.fileOfFileIndex lexbuf.StartPos.FileIndex))
             | "__LINE__" -> KEYWORD_STRING(s, string lexbuf.StartPos.Line)
             | _ -> IdentifierToken args lexbuf s
 

src/Compiler/SyntaxTree/ParseHelpers.fs

@@ -75,7 +75,7 @@ type IParseState with
             match bls.TryGetValue key with
             | true, gen -> gen
             | _ ->
-                let gen = box (SynArgNameGenerator())
+                let gen = !!(box (SynArgNameGenerator()))
                 bls[key] <- gen
                 gen
 
@@ -97,7 +97,7 @@ module LexbufLocalXmlDocStore =
         match lexbuf.BufferLocalStore.TryGetValue xmlDocKey with
         | true, collector -> collector
         | _ ->
-            let collector = box (XmlDocCollector())
+            let collector = !!(box (XmlDocCollector()))
             lexbuf.BufferLocalStore[xmlDocKey] <- collector
             collector
 
@@ -188,7 +188,7 @@ module LexbufIfdefStore =
         match lexbuf.BufferLocalStore.TryGetValue ifDefKey with
         | true, store -> store
         | _ ->
-            let store = box (ResizeArray<ConditionalDirectiveTrivia>())
+            let store = !!(box (ResizeArray<ConditionalDirectiveTrivia>()))
             lexbuf.BufferLocalStore[ifDefKey] <- store
             store
         |> unbox<ResizeArray<ConditionalDirectiveTrivia>>
@@ -237,7 +237,7 @@ module LexbufCommentStore =
         match lexbuf.BufferLocalStore.TryGetValue commentKey with
         | true, store -> store
         | _ ->
-            let store = box (ResizeArray<CommentTrivia>())
+            let store = !!(box (ResizeArray<CommentTrivia>()))
             lexbuf.BufferLocalStore[commentKey] <- store
             store
         |> unbox<ResizeArray<CommentTrivia>>
@@ -894,7 +894,7 @@ let mkRecdField (lidwd: SynLongIdent) = lidwd, true
 // Used for 'do expr' in a class.
 let mkSynDoBinding (vis: SynAccess option, mDo, expr, m) =
     match vis with
-    | Some vis -> errorR (Error(FSComp.SR.parsDoCannotHaveVisibilityDeclarations (vis.ToString()), m))
+    | Some vis -> errorR (Error(FSComp.SR.parsDoCannotHaveVisibilityDeclarations (vis |> string), m))
     | None -> ()
 
     SynBinding(

src/Compiler/SyntaxTree/PrettyNaming.fs

@@ -363,6 +363,8 @@ let IsOperatorDisplayName (name: string) =
 
 let IsPossibleOpName (name: string) = name.StartsWithOrdinal(opNamePrefix)
 
+let ordinalStringComparer: IEqualityComparer<string> = StringComparer.Ordinal
+
 /// Compiles a custom operator into a mangled operator name.
 /// For example, "!%" becomes "op_DereferencePercent".
 /// This function should only be used for custom operators
@@ -387,7 +389,7 @@ let compileCustomOpName =
 
     /// Memoize compilation of custom operators.
     /// They're typically used more than once so this avoids some CPU and GC overhead.
-    let compiledOperators = ConcurrentDictionary<_, string> StringComparer.Ordinal
+    let compiledOperators = ConcurrentDictionary<string, string> ordinalStringComparer
 
     // Cache this as a delegate.
     let compiledOperatorsAddDelegate =
@@ -416,7 +418,7 @@ let compileCustomOpName =
 
 /// Maps the built-in F# operators to their mangled operator names.
 let standardOpNames =
-    let opNames = Dictionary<_, _>(opNameTable.Length, StringComparer.Ordinal)
+    let opNames = Dictionary<_, _>(opNameTable.Length, ordinalStringComparer)
 
     for x, y in opNameTable do
         opNames.Add(x, y)
@@ -440,7 +442,7 @@ let CompileOpName op =
 let decompileCustomOpName =
     // Memoize this operation. Custom operators are typically used more than once
     // so this avoids repeating decompilation.
-    let decompiledOperators = ConcurrentDictionary<_, _> StringComparer.Ordinal
+    let decompiledOperators = ConcurrentDictionary<_, _> ordinalStringComparer
 
     /// The minimum length of the name for a custom operator character.
     /// This value is used when initializing StringBuilders to avoid resizing.
@@ -507,7 +509,7 @@ let decompileCustomOpName =
 
 /// Maps the mangled operator names of built-in F# operators back to the operators.
 let standardOpsDecompile =
-    let ops = Dictionary<string, string>(opNameTable.Length, StringComparer.Ordinal)
+    let ops = Dictionary<string, string>(opNameTable.Length, ordinalStringComparer)
 
     for x, y in opNameTable do
         ops.Add(y, x)
@@ -624,7 +626,7 @@ let IsValidPrefixOperatorUse s =
     if String.IsNullOrEmpty s then
         false
     else
-        match s with
+        match !!s with
         | "?+"
         | "?-"
         | "+"
@@ -635,12 +637,13 @@ let IsValidPrefixOperatorUse s =
         | "%%"
         | "&"
         | "&&" -> true
-        | _ -> s[0] = '!' || isTildeOnlyString s
+        | s -> s[0] = '!' || isTildeOnlyString s
 
 let IsValidPrefixOperatorDefinitionName s =
     if String.IsNullOrEmpty s then
         false
     else
+        let s = !!s
 
         match s[0] with
         | '~' ->
@@ -667,8 +670,8 @@ let IsLogicalPrefixOperator logicalName =
     if String.IsNullOrEmpty logicalName then
         false
     else
-        let displayName = ConvertValLogicalNameToDisplayNameCore logicalName
-        displayName <> logicalName && IsValidPrefixOperatorDefinitionName displayName
+        let displayName = ConvertValLogicalNameToDisplayNameCore !!logicalName
+        displayName <> !!logicalName && IsValidPrefixOperatorDefinitionName displayName
 
 let IsLogicalTernaryOperator logicalName =
     let displayName = ConvertValLogicalNameToDisplayNameCore logicalName

src/Compiler/SyntaxTree/XmlDoc.fs

@@ -79,7 +79,7 @@ type XmlDoc(unprocessedLines: string[], range: range) =
             | Some paramNames ->
 
                 for p in xml.Descendants(XName.op_Implicit "param") do
-                    match p.Attribute(XName.op_Implicit "name") with
+                    match p.Attribute(!!(XName.op_Implicit "name")) with
                     | null -> warning (Error(FSComp.SR.xmlDocMissingParameterName (), doc.Range))
                     | attr ->
                         let nm = attr.Value
@@ -90,9 +90,9 @@ type XmlDoc(unprocessedLines: string[], range: range) =
                 let paramsWithDocs =
                     [
                         for p in xml.Descendants(XName.op_Implicit "param") do
-                            match p.Attribute(XName.op_Implicit "name") with
-                            | null -> ()
-                            | attr -> attr.Value
+                            match p.Attribute(!!(XName.op_Implicit "name")) with
+                            | Null -> ()
+                            | NonNull attr -> attr.Value
                     ]
 
                 if paramsWithDocs.Length > 0 then
@@ -107,7 +107,7 @@ type XmlDoc(unprocessedLines: string[], range: range) =
                     warning (Error(FSComp.SR.xmlDocDuplicateParameter (d), doc.Range))
 
                 for pref in xml.Descendants(XName.op_Implicit "paramref") do
-                    match pref.Attribute(XName.op_Implicit "name") with
+                    match pref.Attribute(!!(XName.op_Implicit "name")) with
                     | null -> warning (Error(FSComp.SR.xmlDocMissingParameterName (), doc.Range))
                     | attr ->
                         let nm = attr.Value
@@ -307,7 +307,7 @@ type XmlDocumentationInfo private (tryGetXmlDocument: unit -> XmlDocument option
             let lines = Array.zeroCreate childNodes.Count
 
             for i = 0 to childNodes.Count - 1 do
-                let childNode = childNodes[i]
+                let childNode = !!childNodes[i]
                 lines[i] <- childNode.OuterXml
 
             XmlDoc(lines, range0))

src/Compiler/TypedTree/QuotationPickler.fs

@@ -249,7 +249,11 @@ let PickleBufferCapacity = 100000
 
 module SimplePickle =
 
-    type Table<'T> =
+    type Table<'T
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    > =
         { tbl: HashMultiMap<'T, int> // This should be "Dictionary"
           mutable rows: 'T list
           mutable count: int }

src/Compiler/TypedTree/TypeProviders.fs

@@ -49,7 +49,7 @@ let GetTypeProviderImplementationTypes (
     // Report an error, blaming the particular type provider component
     let raiseError designTimeAssemblyPathOpt (e: exn) =
         let attrName = typeof<TypeProviderAssemblyAttribute>.Name
-        let exnTypeName = e.GetType().FullName
+        let exnTypeName = !! e.GetType().FullName
         let exnMsg = e.Message
         match designTimeAssemblyPathOpt with 
         | None -> 
@@ -69,16 +69,13 @@ let GetTypeProviderImplementationTypes (
                 [
                     for t in exportedTypes do 
                         let ca = t.GetCustomAttributes(typeof<TypeProviderAttribute>, true)
-                        match ca with 
-                        | Null -> ()
-                        | NonNull ca -> 
-                            if ca.Length > 0 then 
-                                yield t
+                        if ca.Length > 0 then 
+                            yield t
                 ]
             filtered
         with e ->
-            let folder = Path.GetDirectoryName loadedDesignTimeAssembly.Location
-            let exnTypeName = e.GetType().FullName
+            let folder = !! Path.GetDirectoryName(loadedDesignTimeAssembly.Location)
+            let exnTypeName = !! e.GetType().FullName
             let exnMsg = e.Message
             match e with 
             | :? FileLoadException -> 
@@ -92,8 +89,8 @@ let GetTypeProviderImplementationTypes (
 
 let StripException (e: exn) =
     match e with
-    | :? TargetInvocationException as e -> e.InnerException
-    | :? TypeInitializationException as e -> e.InnerException
+    | :? TargetInvocationException as e when isNotNull e.InnerException -> !! e.InnerException
+    | :? TypeInitializationException as e when isNotNull e.InnerException  -> !! e.InnerException
     | _ -> e
 
 /// Create an instance of a type provider from the implementation type for the type provider in the
@@ -116,7 +113,7 @@ let CreateTypeProvider (
             f ()
         with err ->
             let e = StripException (StripException err)
-            raise (TypeProviderError(FSComp.SR.etTypeProviderConstructorException(e.Message), typeProviderImplementationType.FullName, m))
+            raise (TypeProviderError(FSComp.SR.etTypeProviderConstructorException(e.Message), !! typeProviderImplementationType.FullName, m))
 
     let getReferencedAssemblies () =
         resolutionEnvironment.GetReferencedAssemblies() |> Array.distinct
@@ -151,7 +148,7 @@ let CreateTypeProvider (
 
     else
         // No appropriate constructor found
-        raise (TypeProviderError(FSComp.SR.etProviderDoesNotHaveValidConstructor(), typeProviderImplementationType.FullName, m))
+        raise (TypeProviderError(FSComp.SR.etProviderDoesNotHaveValidConstructor(), !! typeProviderImplementationType.FullName, m))
 
 let GetTypeProvidersOfAssembly (
         runtimeAssemblyFilename: string, 
@@ -171,7 +168,7 @@ let GetTypeProvidersOfAssembly (
             let designTimeAssemblyName = 
                 try
                     if designTimeName.EndsWith(".dll", StringComparison.OrdinalIgnoreCase) then
-                        Some (AssemblyName (Path.GetFileNameWithoutExtension designTimeName))
+                        Some (AssemblyName (!!Path.GetFileNameWithoutExtension(designTimeName)))
                     else
                         Some (AssemblyName designTimeName)
                 with :? ArgumentException ->
@@ -193,7 +190,7 @@ let GetTypeProvidersOfAssembly (
                             CreateTypeProvider (t, runtimeAssemblyFilename, resolutionEnvironment, isInvalidationSupported,
                                 isInteractive, systemRuntimeContainsType, systemRuntimeAssemblyVersion, m)
                         match box resolver with 
-                        | Null -> ()
+                        | null -> ()
                         | _ -> yield (resolver, ilScopeRefOfRuntimeAssembly)
 
                 | None, _ -> 
@@ -292,7 +289,7 @@ type ProvidedTypeComparer() =
 
     interface IEqualityComparer<ProvidedType> with
         member _.GetHashCode(ty: ProvidedType) = hash (key ty)
-        member _.Equals(ty1: ProvidedType, ty2: ProvidedType) = (key ty1 = key ty2)
+        member _.Equals(ty1: ProvidedType, ty2: ProvidedType) = nullSafeEquality ty1 ty2 (fun ty1 ty2 -> key ty1 = key ty2)
 
 /// The context used to interpret information in the closure of System.Type, System.MethodInfo and other 
 /// info objects coming from the type provider.
@@ -355,7 +352,7 @@ type ProvidedType (x: Type, ctxt: ProvidedTypeContext) =
     let isMeasure = 
         lazy
             x.CustomAttributes 
-            |> Seq.exists (fun a -> a.Constructor.DeclaringType.FullName = typeof<MeasureAttribute>.FullName)
+            |> Seq.exists (fun a -> (!! a.Constructor.DeclaringType).FullName = typeof<MeasureAttribute>.FullName)
 
     let provide () = ProvidedCustomAttributeProvider (fun _ -> x.CustomAttributes) :> IProvidedCustomAttributeProvider
 
@@ -423,7 +420,7 @@ type ProvidedType (x: Type, ctxt: ProvidedTypeContext) =
 
     member _.GetGenericArguments() = x.GetGenericArguments() |> ProvidedType.CreateArray ctxt
 
-    member _.ApplyStaticArguments(provider: ITypeProvider, fullTypePathAfterArguments, staticArgs: obj[]) = 
+    member _.ApplyStaticArguments(provider: ITypeProvider, fullTypePathAfterArguments, staticArgs: objnull[]) = 
         provider.ApplyStaticArguments(x, fullTypePathAfterArguments,  staticArgs) |> ProvidedType.Create ctxt
 
     member _.IsVoid = (Type.op_Equality(x, typeof<Void>) || (x.Namespace = "System" && x.Name = "Void"))
@@ -484,9 +481,9 @@ type ProvidedType (x: Type, ctxt: ProvidedTypeContext) =
 
     static member CreateNonNull ctxt x = ProvidedType (x, ctxt)
 
-    static member CreateWithNullCheck ctxt name x = 
+    static member CreateWithNullCheck ctxt name (x:Type MaybeNull) = 
         match x with
-        | Null -> nullArg name 
+        | null -> nullArg name 
         | t -> ProvidedType (t, ctxt)
 
     static member CreateArray ctxt (xs: Type[] MaybeNull) : ProvidedType[] MaybeNull = 
@@ -535,7 +532,8 @@ type ProvidedCustomAttributeProvider (attributes :ITypeProvider -> seq<CustomAtt
     [<return: Struct>]
     let (|Arg|_|) (x: CustomAttributeTypedArgument) = match x.Value with null -> ValueNone | v -> ValueSome v
 
-    let findAttribByName tyFullName (a: CustomAttributeData) = (a.Constructor.DeclaringType.FullName = tyFullName)  
+    let findAttribByName tyFullName (a: CustomAttributeData) = ((!!a.Constructor.DeclaringType).FullName = tyFullName)
+
     let findAttrib (ty: Type) a = findAttribByName ty.FullName a
     interface IProvidedCustomAttributeProvider with 
         member _.GetAttributeConstructorArgs (provider, attribName) = 
@@ -545,11 +543,11 @@ type ProvidedCustomAttributeProvider (attributes :ITypeProvider -> seq<CustomAtt
                 let ctorArgs = 
                     a.ConstructorArguments 
                     |> Seq.toList 
-                    |> List.map (function Arg null -> None | Arg obj -> Some obj | _ -> None)
+                    |> List.map (function Arg obj -> Some obj | _ -> None)
                 let namedArgs = 
                     a.NamedArguments 
                     |> Seq.toList 
-                    |> List.map (fun arg -> arg.MemberName, match arg.TypedValue with Arg null -> None | Arg obj -> Some obj | _ -> None)
+                    |> List.map (fun arg -> arg.MemberName, match arg.TypedValue with Arg obj -> Some obj | _ -> None)
                 ctorArgs, namedArgs)
 
         member _.GetHasTypeProviderEditorHideMethodsAttribute provider = 
@@ -608,7 +606,7 @@ type ProvidedMemberInfo (x: MemberInfo, ctxt) =
 type ProvidedParameterInfo (x: ParameterInfo, ctxt) = 
     let provide () = ProvidedCustomAttributeProvider (fun _ -> x.CustomAttributes) :> IProvidedCustomAttributeProvider
 
-    member _.Name = let nm = x.Name in match box nm with null -> "" | _ -> nm
+    member _.Name = let nm = x.Name in match box nm with null -> "" | _ -> !!nm
 
     member _.IsOut = x.IsOut
 
@@ -667,11 +665,11 @@ type ProvidedAssembly (x: Assembly) =
 
     member _.GetName() = x.GetName()
 
-    member _.FullName = x.FullName
+    member _.FullName = !!x.FullName
 
     member _.GetManifestModuleContents(provider: ITypeProvider) = provider.GetGeneratedAssemblyContents x
 
-    static member Create x : ProvidedAssembly MaybeNull = match x with null -> null | t -> ProvidedAssembly (t)
+    static member Create (x: Assembly MaybeNull) : ProvidedAssembly MaybeNull = match x with null -> null | t -> ProvidedAssembly (t)
 
     member _.Handle = x
 
@@ -739,13 +737,13 @@ type ProvidedMethodBase (x: MethodBase, ctxt) =
                         [| typeof<MethodBase> |], null)  
                 if isNull meth then [| |] else
                 let paramsAsObj = 
-                    try meth.Invoke(provider, bindingFlags ||| BindingFlags.InvokeMethod, null, [| box x |], null) 
+                    try (!!meth).Invoke(provider, bindingFlags ||| BindingFlags.InvokeMethod, null, [| box x |], null) 
                     with err -> raise (StripException (StripException err))
                 paramsAsObj :?> ParameterInfo[] 
 
         staticParams |> ProvidedParameterInfo.CreateArrayNonNull ctxt
 
-    member _.ApplyStaticArgumentsForMethod(provider: ITypeProvider, fullNameAfterArguments: string, staticArgs: obj[]) = 
+    member _.ApplyStaticArgumentsForMethod(provider: ITypeProvider, fullNameAfterArguments: string, staticArgs: objnull[]) = 
         let bindingFlags = BindingFlags.Instance ||| BindingFlags.Public ||| BindingFlags.InvokeMethod
 
         let mb = 
@@ -760,8 +758,9 @@ type ProvidedMethodBase (x: MethodBase, ctxt) =
                         [| typeof<MethodBase>; typeof<string>; typeof<obj[]> |], null)  
 
                 match meth with 
-                | Null -> failwith (FSComp.SR.estApplyStaticArgumentsForMethodNotImplemented())
-                | _ -> 
+                | null -> failwith (FSComp.SR.estApplyStaticArgumentsForMethodNotImplemented())
+                | meth -> 
+
                 let mbAsObj = 
                     try meth.Invoke(provider, bindingFlags ||| BindingFlags.InvokeMethod, null, [| box x; box fullNameAfterArguments; box staticArgs  |], null) 
                     with err -> raise (StripException (StripException err))
@@ -844,6 +843,7 @@ type ProvidedMethodInfo (x: MethodInfo, ctxt) =
         | Null -> null
         | NonNull x -> ProvidedMethodInfo (x, ctxt)
 
+
     static member CreateArray ctxt (xs: MethodInfo[] MaybeNull) : ProvidedMethodInfo[] MaybeNull = 
         match xs with 
         | Null -> null
@@ -1096,14 +1096,14 @@ let GetInvokerExpression (provider: ITypeProvider, methodBase: ProvidedMethodBas
 
 /// Compute the Name or FullName property of a provided type, reporting appropriate errors
 let CheckAndComputeProvidedNameProperty(m, st: Tainted<ProvidedType>, proj, propertyString) =
-    let name = 
+    let name : string MaybeNull = 
         try st.PUntaint(proj, m) 
         with :? TypeProviderError as tpe -> 
             let newError = tpe.MapText((fun msg -> FSComp.SR.etProvidedTypeWithNameException(propertyString, msg)), st.TypeProviderDesignation, m)
             raise newError
     if String.IsNullOrEmpty name then
         raise (TypeProviderError(FSComp.SR.etProvidedTypeWithNullOrEmptyName propertyString, st.TypeProviderDesignation, m))
-    name
+    !!name
 
 /// Verify that this type provider has supported attributes
 let ValidateAttributesOfProvidedType (m, st: Tainted<ProvidedType>) =         
@@ -1191,7 +1191,7 @@ let ValidateProvidedTypeAfterStaticInstantiation(m, st: Tainted<ProvidedType>, e
                     let miDeclaringTypeFullName = 
                         TryMemberMember (miDeclaringType, fullName, memberName, "FullName", m,
                             "invalid declaring type full name",
-                            fun miDeclaringType -> miDeclaringType.FullName)
+                            fun miDeclaringType -> !!miDeclaringType.FullName)
                         |> unmarshal
 
                     if not (ProvidedType.TaintedEquals (st, miDeclaringType)) then 
@@ -1345,7 +1345,7 @@ let ComputeMangledNameForApplyStaticParameters(nm, staticArgs, staticParams: Tai
     PrettyNaming.ComputeMangledNameWithoutDefaultArgValues(nm, staticArgs, defaultArgValues)
 
 /// Apply the given provided method to the given static arguments (the arguments are assumed to have been sorted into application order)
-let TryApplyProvidedMethod(methBeforeArgs: Tainted<ProvidedMethodBase>, staticArgs: obj[], m: range) =
+let TryApplyProvidedMethod(methBeforeArgs: Tainted<ProvidedMethodBase>, staticArgs: objnull[], m: range) =
     if staticArgs.Length = 0 then 
         Some methBeforeArgs
     else
@@ -1364,7 +1364,7 @@ let TryApplyProvidedMethod(methBeforeArgs: Tainted<ProvidedMethodBase>, staticAr
 
 
 /// Apply the given provided type to the given static arguments (the arguments are assumed to have been sorted into application order
-let TryApplyProvidedType(typeBeforeArguments: Tainted<ProvidedType>, optGeneratedTypePath: string list option, staticArgs: obj[], m: range) =
+let TryApplyProvidedType(typeBeforeArguments: Tainted<ProvidedType>, optGeneratedTypePath: string list option, staticArgs: objnull[], m: range) =
     if staticArgs.Length = 0 then 
         Some (typeBeforeArguments, (fun () -> ()))
     else 
@@ -1427,7 +1427,7 @@ let TryLinkProvidedType(resolver: Tainted<ITypeProvider>, moduleOrNamespace: str
                         sp.PUntaint((fun sp -> 
                             let pt = sp.ParameterType
                             let uet = if pt.IsEnum then pt.GetEnumUnderlyingType() else pt
-                            uet.FullName), range)
+                            !!uet.FullName), range)
 
                     match spReprTypeName with 
                     | "System.SByte" -> box (sbyte arg)
@@ -1449,8 +1449,8 @@ let TryLinkProvidedType(resolver: Tainted<ITypeProvider>, moduleOrNamespace: str
                 | _ ->
                     if sp.PUntaint ((fun sp -> sp.IsOptional), range) then 
                         match sp.PUntaint((fun sp -> sp.RawDefaultValue), range) with
-                        | Null -> error (Error(FSComp.SR.etStaticParameterRequiresAValue (spName, typeBeforeArgumentsName, typeBeforeArgumentsName, spName), range0))
-                        | NonNull v -> v
+                        | null -> error (Error(FSComp.SR.etStaticParameterRequiresAValue (spName, typeBeforeArgumentsName, typeBeforeArgumentsName, spName), range0))
+                        | v -> v
                     else
                         error(Error(FSComp.SR.etProvidedTypeReferenceMissingArgument spName, range0)))
                 
@@ -1467,7 +1467,7 @@ let GetPartsOfNamespaceRecover(namespaceName: string MaybeNull) =
     | Null -> [] 
     | NonNull namespaceName -> 
         if namespaceName.Length = 0 then ["<NonExistentNamespace>"]
-        else splitNamespace (nonNull namespaceName)
+        else splitNamespace namespaceName
 
 /// Get the parts of a .NET namespace. Special rules: null means global, empty is not allowed.
 let GetProvidedNamespaceAsPath (m, resolver: Tainted<ITypeProvider>, namespaceName:string MaybeNull) =

src/Compiler/TypedTree/TypeProviders.fsi

@@ -214,7 +214,7 @@ type ProvidedType =
 type IProvidedCustomAttributeProvider =
     abstract GetHasTypeProviderEditorHideMethodsAttribute: provider: ITypeProvider -> bool
 
-    abstract GetDefinitionLocationAttribute: provider: ITypeProvider -> (string * int * int) option
+    abstract GetDefinitionLocationAttribute: provider: ITypeProvider -> (string MaybeNull * int * int) option
 
     abstract GetXmlDocAttributes: provider: ITypeProvider -> string[]
 
@@ -313,7 +313,7 @@ type ProvidedParameterInfo =
 
     member IsOptional: bool
 
-    member RawDefaultValue: obj
+    member RawDefaultValue: objnull
 
     member HasDefaultValue: bool
 
@@ -482,12 +482,12 @@ val ValidateProvidedTypeAfterStaticInstantiation:
 /// to check the type name is as expected (this function is called by the caller of TryApplyProvidedType
 /// after other checks are made).
 val TryApplyProvidedType:
-    typeBeforeArguments: Tainted<ProvidedType> * optGeneratedTypePath: string list option * staticArgs: obj[] * range ->
+    typeBeforeArguments: Tainted<ProvidedType> * optGeneratedTypePath: string list option * staticArgs: objnull[] * range ->
         (Tainted<ProvidedType> * (unit -> unit)) option
 
 /// Try to apply a provided method to the given static arguments.
 val TryApplyProvidedMethod:
-    methBeforeArgs: Tainted<ProvidedMethodBase> * staticArgs: obj[] * range -> Tainted<ProvidedMethodBase> option
+    methBeforeArgs: Tainted<ProvidedMethodBase> * staticArgs: objnull[] * range -> Tainted<ProvidedMethodBase> option
 
 /// Try to resolve a type in the given extension type resolver
 val TryResolveProvidedType: Tainted<ITypeProvider> * range * string[] * typeName: string -> Tainted<ProvidedType> option

src/Compiler/TypedTree/TypedTree.fs

@@ -3392,7 +3392,7 @@ type NonLocalValOrMemberRef =
     member x.DebugText = x.ToString()
 
     /// For debugging
-    override x.ToString() = x.EnclosingEntity.nlr.ToString() + "::" + x.ItemKey.PartialKey.LogicalName
+    override x.ToString() = !! x.EnclosingEntity.nlr.ToString() + "::" + x.ItemKey.PartialKey.LogicalName
       
 /// Represents the path information for a reference to a value or member in another assembly, disassociated
 /// from any particular reference.
@@ -5112,11 +5112,11 @@ type Expr =
 
     override expr.ToString() = expr.ToDebugString(3)
 
-    member expr.ToDebugString(depth: int) = 
+    member expr.ToDebugString(depth: int) : string = 
         if depth = 0 then ".." else
         let depth = depth - 1
         match expr with 
-        | Const (c, _, _) -> c.ToString()
+        | Const (c, _, _) -> string c
         | Val (v, _, _) -> v.LogicalName
         | Sequential (e1, e2, _, _) -> "Sequential(" + e1.ToDebugString(depth) + ", " + e2.ToDebugString(depth) + ")"
         | Lambda (_, _, _, vs, body, _, _) -> sprintf "Lambda(%+A, " vs + body.ToDebugString(depth) + ")" 
@@ -5695,13 +5695,13 @@ module CcuTypeForwarderTable =
             if remainingPath.Count = 0 then
                 finalKey
             else
-                remainingPath.Array.[remainingPath.Offset]
+                (!!remainingPath.Array).[remainingPath.Offset]
         match nodes.TryGetValue searchTerm with
         | true, innerTree ->
             if remainingPath.Count = 0 then
                 innerTree.Value
             else
-                 findInTree (ArraySegment<string>(remainingPath.Array, remainingPath.Offset + 1, remainingPath.Count - 1)) finalKey innerTree
+                 findInTree (ArraySegment<string>((!!remainingPath.Array), remainingPath.Offset + 1, remainingPath.Count - 1)) finalKey innerTree
         | false, _ -> None
 
 /// Represents a table of .NET CLI type forwarders for an assembly
@@ -6011,7 +6011,7 @@ type Construct() =
         let lazyBaseTy = 
             LazyWithContext.Create 
                 ((fun (m, objTy) -> 
-                      let baseSystemTy = st.PApplyOption((fun st -> match st.BaseType with null -> None | ty -> Some (nonNull ty)), m)
+                      let baseSystemTy = st.PApplyOption((fun st -> match st.BaseType with null -> None | ty -> Some (ty)), m)
                       match baseSystemTy with 
                       | None -> objTy 
                       | Some t -> importProvidedType t),
@@ -6334,5 +6334,5 @@ type Construct() =
             // Coordinates from type provider are 1-based for lines and columns
             // Coordinates internally in the F# compiler are 1-based for lines and 0-based for columns
             let pos = Position.mkPos line (max 0 (column - 1)) 
-            mkRange filePath pos pos |> Some
+            mkRange !!filePath pos pos |> Some
 #endif

src/Compiler/TypedTree/TypedTree.fsi

@@ -3139,7 +3139,7 @@ type TType =
 
     /// For now, used only as a discriminant in error message.
     /// See https://github.com/dotnet/fsharp/issues/2561
-    member GetAssemblyName: unit -> string
+    member GetAssemblyName: unit -> string MaybeNull
 
     override ToString: unit -> string
 

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -38,6 +38,19 @@ let AccFreeVarsStackGuardDepth = GetEnvInteger "FSHARP_AccFreeVars" 100
 let RemapExprStackGuardDepth = GetEnvInteger "FSHARP_RemapExpr" 50
 let FoldExprStackGuardDepth = GetEnvInteger "FSHARP_FoldExpr" 50
 
+let inline compareBy (x: 'T MaybeNull) (y: 'T MaybeNull) ([<InlineIfLambda>]func: 'T -> 'K)  = 
+#if NO_CHECKNULLS
+    compare (func x) (func y)
+#else
+    match x,y with
+    | null,null -> 0
+    | null,_ -> -1
+    | _,null -> 1
+    | x,y ->  compare (func !!x) (func !!y)
+#endif
+
+
+
 //---------------------------------------------------------------------------
 // Basic data structures
 //---------------------------------------------------------------------------
@@ -1187,9 +1200,9 @@ let rec getErasedTypes g ty checkForNullness =
 // Standard orderings, e.g. for order set/map keys
 //---------------------------------------------------------------------------
 
-let valOrder = { new IComparer<Val> with member _.Compare(v1, v2) = compare v1.Stamp v2.Stamp }
+let valOrder = { new IComparer<Val> with member _.Compare(v1, v2) = compareBy v1 v2 _.Stamp }
 
-let tyconOrder = { new IComparer<Tycon> with member _.Compare(tycon1, tycon2) = compare tycon1.Stamp tycon2.Stamp }
+let tyconOrder = { new IComparer<Tycon> with member _.Compare(tycon1, tycon2) = compareBy tycon1 tycon2 _.Stamp }
 
 let recdFieldRefOrder = 
     { new IComparer<RecdFieldRef> with 
@@ -2168,7 +2181,7 @@ let unionFreeTycons s1 s2 =
 
 let typarOrder = 
     { new IComparer<Typar> with 
-        member x.Compare (v1: Typar, v2: Typar) = compare v1.Stamp v2.Stamp } 
+        member x.Compare (v1: Typar, v2: Typar) = compareBy v1 v2 _.Stamp } 
 
 let emptyFreeTypars = Zset.empty typarOrder
 let unionFreeTypars s1 s2 = 
@@ -3230,7 +3243,7 @@ type DisplayEnv =
               ControlPath
               (splitNamespace ExtraTopLevelOperatorsName) ]
 
-let (+.+) s1 s2 = if String.IsNullOrEmpty(s1) then s2 else s1+"."+s2
+let (+.+) s1 s2 = if String.IsNullOrEmpty(s1) then s2 else !!s1+"."+s2
 
 let layoutOfPath p =
     sepListL SepL.dot (List.map (tagNamespace >> wordL) p)
@@ -6236,7 +6249,7 @@ and remapTyconRepr ctxt tmenv repr =
                  // This is actually done on-demand (see the implementation of ProvidedTypeContext)
                  ProvidedType = 
                      info.ProvidedType.PApplyNoFailure (fun st -> 
-                         let ctxt = st.Context.RemapTyconRefs(unbox >> remapTyconRef tmenv.tyconRefRemap >> box) 
+                         let ctxt = st.Context.RemapTyconRefs(unbox >> remapTyconRef tmenv.tyconRefRemap >> box >> (!!)) 
                          ProvidedType.ApplyContext (st, ctxt)) }
 #endif
     | TNoRepr -> repr
@@ -8210,7 +8223,7 @@ let mkCompilationMappingAttrForQuotationResource (g: TcGlobals) (nm, tys: ILType
 #if !NO_TYPEPROVIDERS
 
 let isTypeProviderAssemblyAttr (cattr: ILAttribute) = 
-    cattr.Method.DeclaringType.BasicQualifiedName = typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName
+    cattr.Method.DeclaringType.BasicQualifiedName = !! typeof<Microsoft.FSharp.Core.CompilerServices.TypeProviderAssemblyAttribute>.FullName
 
 let TryDecodeTypeProviderAssemblyAttr (cattr: ILAttribute) : string MaybeNull option = 
     if isTypeProviderAssemblyAttr cattr then 
@@ -8985,7 +8998,7 @@ let buildAccessPath (cp: CompilationPath option) =
         System.String.Join(".", ap)      
     | None -> "Extension Type"
 
-let prependPath path name = if String.IsNullOrEmpty(path) then name else path + "." + name
+let prependPath path name = if String.IsNullOrEmpty(path) then name else !!path + "." + name
 
 let XmlDocSigOfVal g full path (v: Val) =
     let parentTypars, methTypars, cxs, argInfos, retTy, prefix, path, name = 
@@ -11302,7 +11315,7 @@ type TraitWitnessInfoHashMap<'T> = ImmutableDictionary<TraitWitnessInfo, 'T>
 let EmptyTraitWitnessInfoHashMap g : TraitWitnessInfoHashMap<'T> =
     ImmutableDictionary.Create(
         { new IEqualityComparer<_> with 
-            member _.Equals(a, b) = traitKeysAEquiv g TypeEquivEnv.Empty a b
+            member _.Equals(a, b) = nullSafeEquality a b (fun a b -> traitKeysAEquiv g TypeEquivEnv.Empty a b)
             member _.GetHashCode(a) = hash a.MemberName
         })
 

src/Compiler/TypedTree/TypedTreePickle.fs

@@ -67,7 +67,11 @@ type PickledDataWithReferences<'rawData> =
 //---------------------------------------------------------------------------
 
 [<NoEquality; NoComparison>]
+#if NO_CHECKNULLS
 type Table<'T> =
+#else
+type Table<'T when 'T: not null> =
+#endif
     { name: string
       tbl: Dictionary<'T, int>
       mutable rows: ResizeArray<'T>

src/Compiler/TypedTree/tainted.fs

@@ -89,7 +89,7 @@ type internal Tainted<'T> (context: TaintedContext, value: 'T) =
         | _ -> ()
 
     member _.TypeProviderDesignation = 
-        context.TypeProvider.GetType().FullName
+        !! context.TypeProvider.GetType().FullName
 
     member _.TypeProviderAssemblyRef = 
         context.TypeProviderAssemblyRef

src/Compiler/Utilities/Activity.fs

@@ -6,6 +6,8 @@ open System
 open System.Diagnostics
 open System.IO
 open System.Text
+open Internal.Utilities.Library
+
 
 module ActivityNames =
     [<Literal>]
@@ -137,16 +139,17 @@ module internal Activity =
                     ActivityStarted = (fun a -> a.AddTag(gcStatsInnerTag, collectGCStats ()) |> ignore),
                     ActivityStopped =
                         (fun a ->
-                            let statsBefore = a.GetTagItem(gcStatsInnerTag) :?> GCStats
                             let statsAfter = collectGCStats ()
                             let p = Process.GetCurrentProcess()
                             a.AddTag(Tags.workingSetMB, p.WorkingSet64 / 1_000_000L) |> ignore
                             a.AddTag(Tags.handles, p.HandleCount) |> ignore
                             a.AddTag(Tags.threads, p.Threads.Count) |> ignore
 
-                            for i = 0 to statsAfter.Length - 1 do
-                                a.AddTag($"gc{i}", statsAfter[i] - statsBefore[i]) |> ignore)
-
+                            match a.GetTagItem(gcStatsInnerTag) with
+                            | :? GCStats as statsBefore ->
+                                for i = 0 to statsAfter.Length - 1 do
+                                    a.AddTag($"gc{i}", statsAfter[i] - statsBefore[i]) |> ignore
+                            | _ -> ())
                 )
 
             ActivitySource.AddActivityListener(l)
@@ -197,11 +200,11 @@ module internal Activity =
 
     module CsvExport =
 
-        let private escapeStringForCsv (o: obj) =
-            if isNull o then
-                ""
-            else
-                let mutable txtVal = o.ToString()
+        let private escapeStringForCsv (o: obj MaybeNull) =
+            match o with
+            | null -> ""
+            | o ->
+                let mutable txtVal = match o.ToString() with | null -> "" | s -> s
                 let hasComma = txtVal.IndexOf(',') > -1
                 let hasQuote = txtVal.IndexOf('"') > -1
 
@@ -216,7 +219,7 @@ module internal Activity =
         let private createCsvRow (a: Activity) =
             let sb = new StringBuilder(128)
 
-            let appendWithLeadingComma (s: string) =
+            let appendWithLeadingComma (s: string MaybeNull) =
                 sb.Append(',') |> ignore
                 sb.Append(s) |> ignore
 
@@ -234,7 +237,7 @@ module internal Activity =
 
             sb.ToString()
 
-        let addCsvFileListener pathToFile =
+        let addCsvFileListener (pathToFile:string) =
             if pathToFile |> File.Exists |> not then
                 File.WriteAllLines(
                     pathToFile,
@@ -256,7 +259,7 @@ module internal Activity =
 
             let l =
                 new ActivityListener(
-                    ShouldListenTo = (fun a -> ActivityNames.AllRelevantNames |> Array.contains a.Name),
+                    ShouldListenTo = (fun a ->ActivityNames.AllRelevantNames |> Array.contains a.Name),
                     Sample = (fun _ -> ActivitySamplingResult.AllData),
                     ActivityStopped = (fun a -> msgQueue.Post(createCsvRow a))
                 )

src/Compiler/Utilities/FileSystem.fs

@@ -427,7 +427,7 @@ module internal FileSystemUtils =
             if not (hasExtensionWithValidate false path) then
                 raise (ArgumentException("chopExtension")) // message has to be precisely this, for OCaml compatibility, and no argument name can be set
 
-            Path.Combine(Path.GetDirectoryName path, Path.GetFileNameWithoutExtension(path))
+            Path.Combine(!! Path.GetDirectoryName(path), !! Path.GetFileNameWithoutExtension(path))
 
     let fileNameOfPath path =
         checkPathForIllegalChars path
@@ -694,15 +694,19 @@ type DefaultFileSystem() as this =
     default _.IsStableFileHeuristic(fileName: string) =
         let directory = Path.GetDirectoryName fileName
 
-        directory.Contains("Reference Assemblies/")
-        || directory.Contains("Reference Assemblies\\")
-        || directory.Contains("packages/")
-        || directory.Contains("packages\\")
-        || directory.Contains("lib/mono/")
+        match directory with
+        | Null -> false
+        | NonNull directory ->
+            directory.Contains("Reference Assemblies/")
+            || directory.Contains("Reference Assemblies\\")
+            || directory.Contains("packages/")
+            || directory.Contains("packages\\")
+            || directory.Contains("lib/mono/")
 
     abstract ChangeExtensionShim: path: string * extension: string -> string
 
-    default _.ChangeExtensionShim(path: string, extension: string) : string = Path.ChangeExtension(path, extension)
+    default _.ChangeExtensionShim(path: string, extension: string) : string =
+        !! Path.ChangeExtension(path, extension)
 
     interface IFileSystem with
         member _.AssemblyLoader = this.AssemblyLoader
@@ -820,7 +824,7 @@ module public StreamExtensions =
                 use sr = new StreamReader(s, encoding, true)
 
                 while not <| sr.EndOfStream do
-                    yield sr.ReadLine()
+                    yield !! sr.ReadLine()
             }
 
         member s.ReadAllLines(?encoding: Encoding) : string array =

src/Compiler/Utilities/HashMultiMap.fs

@@ -7,7 +7,11 @@ open System.Collections.Generic
 // Each entry in the HashMultiMap dictionary has at least one entry. Under normal usage each entry has _only_
 // one entry. So use two hash tables: one for the main entries and one for the overflow.
 [<Sealed>]
-type internal HashMultiMap<'Key, 'Value>(size: int, comparer: IEqualityComparer<'Key>) =
+type internal HashMultiMap<'Key, 'Value
+#if !NO_CHECKNULLS
+    when 'Key:not null
+#endif
+    >(size: int, comparer: IEqualityComparer<'Key>) =
 
     let firstEntries = Dictionary<_, _>(size, comparer)
 

src/Compiler/Utilities/HashMultiMap.fsi

@@ -7,7 +7,11 @@ open System.Collections.Generic
 /// Hash tables, by default based on F# structural "hash" and (=) functions.
 /// The table may map a single key to multiple bindings.
 [<Sealed>]
-type internal HashMultiMap<'Key, 'Value> =
+type internal HashMultiMap<'Key, 'Value
+#if !NO_CHECKNULLS
+    when 'Key:not null
+#endif
+    > =
     /// Create a new empty mutable HashMultiMap with the given key hash/equality functions.
     new: comparer: IEqualityComparer<'Key> -> HashMultiMap<'Key, 'Value>
 

src/Compiler/Utilities/LruCache.fs

@@ -6,6 +6,7 @@ open System
 open System.Collections.Generic
 open System.Diagnostics
 
+open Internal.Utilities.Library
 open Internal.Utilities.Library.Extras
 
 [<RequireQualifiedAccess>]
@@ -22,7 +23,12 @@ type internal ValueLink<'T when 'T: not struct> =
     | Weak of WeakReference<'T>
 
 [<DebuggerDisplay("{DebuggerDisplay}")>]
-type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality and 'TValue: not struct>
+type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality and 'TValue: not struct
+#if !NO_CHECKNULLS
+    and 'TKey:not null
+    and 'TVersion:not null
+#endif
+    >
     (keepStrongly, ?keepWeakly, ?requiredToKeep, ?event) =
 
     let keepWeakly = defaultArg keepWeakly 100
@@ -35,8 +41,10 @@ type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVers
     let strongList = LinkedList<'TKey * 'TVersion * string * ValueLink<'TValue>>()
     let weakList = LinkedList<'TKey * 'TVersion * string * ValueLink<'TValue>>()
 
-    let rec removeCollected (node: LinkedListNode<_>) =
-        if node <> null then
+    let rec removeCollected (possiblyNullNode: LinkedListNode<_> MaybeNull) =
+        match possiblyNullNode with
+        | null -> ()
+        | node ->
             let key, version, label, value = node.Value
 
             match value with
@@ -64,9 +72,10 @@ type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVers
             let mutable node = weakList.Last
 
             while weakList.Count > keepWeakly && node <> null do
-                let previous = node.Previous
-                let key, version, label, _ = node.Value
-                weakList.Remove node
+                let notNullNode = !! node
+                let previous = notNullNode.Previous
+                let key, version, label, _ = notNullNode.Value
+                weakList.Remove notNullNode
                 dictionary[key].Remove version |> ignore
 
                 if dictionary[key].Count = 0 then
@@ -81,14 +90,15 @@ type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVers
         let mutable anythingWeakened = false
 
         while strongList.Count > keepStrongly && node <> null do
-            let previous = node.Previous
+            let notNullNode = !! node
+            let previous = notNullNode.Previous
 
-            match node.Value with
+            match notNullNode.Value with
             | _, _, _, Strong v when requiredToKeep v -> ()
             | key, version, label, Strong v ->
-                strongList.Remove node
-                node.Value <- key, version, label, Weak(WeakReference<_> v)
-                weakList.AddFirst node
+                strongList.Remove notNullNode
+                notNullNode.Value <- key, version, label, Weak(WeakReference<_> v)
+                weakList.AddFirst notNullNode
                 event CacheEvent.Weakened (label, key, version)
                 anythingWeakened <- true
             | _key, _version, _label, _ -> failwith "Invalid state, weak reference in strong list"

src/Compiler/Utilities/LruCache.fsi

@@ -12,7 +12,12 @@ type internal CacheEvent =
 ///
 /// It's also versioned, meaning each key can have multiple versions and only the latest one is kept strongly.
 /// Older versions are kept weakly and can be collected by GC.
-type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality and 'TValue: not struct> =
+type internal LruCache<'TKey, 'TVersion, 'TValue when 'TKey: equality and 'TVersion: equality and 'TValue: not struct
+#if !NO_CHECKNULLS
+    and 'TKey:not null
+    and 'TVersion:not null
+#endif
+    > =
 
     /// <param name="keepStrongly">Maximum number of strongly held results to keep in the cache</param>
     /// <param name="keepWeakly">Maximum number of weakly held results to keep in the cache</param>

src/Compiler/Utilities/NullnessShims.fs

@@ -0,0 +1,72 @@
+namespace Internal.Utilities.Library
+
+open System
+
+[<AutoOpen>]
+module internal NullnessShims =
+
+    let inline isNotNull (x: 'T) = not (isNull x)
+
+#if NO_CHECKNULLS || BUILDING_WITH_LKG
+    type 'T MaybeNull when 'T: not struct = 'T
+    type objnull = obj
+
+    let inline (^) (a: 'a) ([<InlineIfLambda>] b: 'a -> 'b) : 'b =
+        match a with
+        | null -> Unchecked.defaultof<'b>
+        | _ -> b a
+
+    let inline (|NonNullQuick|) (x: 'T MaybeNull) =
+        match x with
+        | null -> raise (NullReferenceException())
+        | v -> v
+
+    let inline nonNull<'T when 'T:not struct and 'T:null> (x: 'T MaybeNull ) =
+        match x with
+        | null -> raise (NullReferenceException())
+        | v -> v
+
+    let inline (|Null|NonNull|) (x: 'T MaybeNull) : Choice<unit, 'T> =
+        match x with
+        | null -> Null
+        | v -> NonNull v
+
+    let inline nullArgCheck paramName (x: 'T MaybeNull) =
+        if isNull (box x) then raise (ArgumentNullException(paramName))
+        else x
+
+    let inline (!!) x = x
+
+    let inline defaultIfNull defaultValue arg = match arg with | null -> defaultValue | _ -> arg
+
+    let inline nullSafeEquality (x: MaybeNull<'T>) (y: MaybeNull<'T>) ([<InlineIfLambda>]nonNullEqualityFunc:'T->'T->bool) =
+        match box x, box y with
+        | null, null -> true
+        | null,_ | _, null -> false
+        | _,_ -> nonNullEqualityFunc x y
+#else
+    type 'T MaybeNull when 'T: not null and 'T: not struct = 'T | null
+
+    let inline (^) (a: 'a | null) ([<InlineIfLambda>] b: 'a -> 'b) : ('b | null) =
+        match a with
+        | Null -> null
+        | NonNull v -> b v
+
+    let inline (!!) (x:'T | null) = Unchecked.nonNull x
+
+    let inline nullSafeEquality (x: MaybeNull<'T>) (y: MaybeNull<'T>) ([<InlineIfLambda>]nonNullEqualityFunc:'T->'T->bool) =
+        match x, y with
+        | null, null -> true
+        | null,_ | _, null -> false
+        | x, y -> nonNullEqualityFunc !!x !!y
+
+#endif    
+
+
+
+    [<return:Struct>]
+    let inline (|NonEmptyString|_|) (x: string MaybeNull) =
+        match x with
+        | null -> ValueNone
+        | "" -> ValueNone
+        | v -> ValueSome v

src/Compiler/Utilities/TaggedCollections.fs

@@ -9,6 +9,7 @@ namespace Internal.Utilities.Collections.Tagged
 open Microsoft.FSharp.Core
 open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
 open System.Collections.Generic
+open Internal.Utilities.Library
 
 [<NoEquality; NoComparison>]
 [<AllowNullLiteral>]
@@ -141,7 +142,7 @@ module SetTree =
                 | _ -> add comparer k (add comparer t2.Key t1)
             | _ -> add comparer k (add comparer t1.Key t2)
 
-    let rec split (comparer: IComparer<'T>) pivot (t: SetTree<'T>) =
+    let rec split (comparer: IComparer<'T>) (pivot: 'T) (t: SetTree<'T>) =
         // Given a pivot and a set t
         // Return { x in t s.t. x < pivot }, pivot in t?, { x in t s.t. x > pivot }
         if isEmpty t then
@@ -178,7 +179,7 @@ module SetTree =
                     let k3, l' = spliceOutSuccessor tn.Left in k3, mk l' tn.Key tn.Right
             | _ -> t.Key, empty
 
-    let rec remove (comparer: IComparer<'T>) k (t: SetTree<'T>) =
+    let rec remove (comparer: IComparer<'T>) (k: 'T) (t: SetTree<'T>) =
         if isEmpty t then
             t
         else
@@ -200,7 +201,7 @@ module SetTree =
                     rebalance tn.Left tn.Key (remove comparer k tn.Right)
             | _ -> if c = 0 then empty else t
 
-    let rec contains (comparer: IComparer<'T>) k (t: SetTree<'T>) =
+    let rec contains (comparer: IComparer<'T>) (k: 'T) (t: SetTree<'T>) =
         if isEmpty t then
             false
         else
@@ -809,7 +810,7 @@ module MapTree =
     let indexNotFound () =
         raise (KeyNotFoundException("An index satisfying the predicate was not found in the collection"))
 
-    let rec tryGetValue (comparer: IComparer<'Key>) k (v: byref<'Value>) (m: MapTree<'Key, 'Value>) =
+    let rec tryGetValue (comparer: IComparer<'Key>) (k: 'Key) (v: byref<'Value>) (m: MapTree<'Key, 'Value>) =
         if isEmpty m then
             false
         else
@@ -823,7 +824,7 @@ module MapTree =
                 | :? MapTreeNode<'Key, 'Value> as mn -> tryGetValue comparer k &v (if c < 0 then mn.Left else mn.Right)
                 | _ -> false
 
-    let find (comparer: IComparer<'Key>) k (m: MapTree<'Key, 'Value>) =
+    let find (comparer: IComparer<'Key>) (k: 'Key) (m: MapTree<'Key, 'Value>) =
         let mutable v = Unchecked.defaultof<'Value>
 
         if tryGetValue comparer k &v m then v else indexNotFound ()

src/Compiler/Utilities/illib.fs

@@ -17,7 +17,8 @@ type InterruptibleLazy<'T> private (value, valueFactory: unit -> 'T) =
 
     [<VolatileField>]
     // TODO nullness - this is boxed to obj because of an attribute targets bug fixed in main, but not yet shipped (needs shipped 8.0.400)
-    let mutable valueFactory : obj = valueFactory
+    let mutable valueFactory : objnull = valueFactory
+
 
     let mutable value = value
 
@@ -86,35 +87,6 @@ module internal PervasiveAutoOpens =
         | [ _ ] -> true
         | _ -> false
 
-    let inline isNotNull (x: 'T) = not (isNull x)
-
-#if NO_CHECKNULLS
-    type 'T MaybeNull when 'T: null and 'T: not struct = 'T
-
-    let inline (|NonNullQuick|) (x: 'T MaybeNull) =
-        match x with
-        | null -> raise (NullReferenceException())
-        | v -> v
-
-    let inline nonNull (x: 'T MaybeNull) =
-        match x with
-        | null -> raise (NullReferenceException())
-        | v -> v
-
-    let inline (|Null|NonNull|) (x: 'T MaybeNull) : Choice<unit, 'T> =
-        match x with
-        | null -> Null
-        | v -> NonNull v
-
-    let inline nullArgCheck paramName (x: 'T MaybeNull) =
-        match x with
-        | null -> raise (ArgumentNullException(paramName))
-        | v -> v
-#else
-    type 'T MaybeNull when 'T: not null and 'T: not struct = 'T | null
-
-#endif
-
     let inline (===) x y = LanguagePrimitives.PhysicalEquality x y
 
     /// Per the docs the threshold for the Large Object Heap is 85000 bytes: https://learn.microsoft.com/dotnet/standard/garbage-collection/large-object-heap#how-an-object-ends-up-on-the-large-object-heap-and-how-gc-handles-them
@@ -132,7 +104,7 @@ module internal PervasiveAutoOpens =
         member inline x.EndsWithOrdinalIgnoreCase value =
             x.EndsWith(value, StringComparison.OrdinalIgnoreCase)
 
-        member inline x.IndexOfOrdinal value =
+        member inline x.IndexOfOrdinal (value:string) =
             x.IndexOf(value, StringComparison.Ordinal)
 
         member inline x.IndexOfOrdinal(value, startIndex) =
@@ -182,8 +154,8 @@ module internal PervasiveAutoOpens =
 type DelayInitArrayMap<'T, 'TDictKey, 'TDictValue>(f: unit -> 'T[]) =
     let syncObj = obj ()
 
-    let mutable arrayStore = null
-    let mutable dictStore = null
+    let mutable arrayStore : _ array MaybeNull = null
+    let mutable dictStore : _ MaybeNull = null
 
     let mutable func = f
 
@@ -197,11 +169,11 @@ type DelayInitArrayMap<'T, 'TDictKey, 'TDictValue>(f: unit -> 'T[]) =
                 match arrayStore with
                 | NonNull value -> value
                 | _ ->
-
-                    arrayStore <- func ()
+                    let freshArray = func ()
+                    arrayStore <- freshArray
 
                     func <- Unchecked.defaultof<_>
-                    arrayStore
+                    freshArray
             finally
                 Monitor.Exit(syncObj)
 
@@ -216,9 +188,9 @@ type DelayInitArrayMap<'T, 'TDictKey, 'TDictValue>(f: unit -> 'T[]) =
                 match dictStore with
                 | NonNull value -> value
                 | _ ->
-
-                    dictStore <- this.CreateDictionary(array)
-                    dictStore
+                    let dict = this.CreateDictionary(array)
+                    dictStore <- dict
+                    dict
             finally
                 Monitor.Exit(syncObj)
 
@@ -231,7 +203,7 @@ type DelayInitArrayMap<'T, 'TDictKey, 'TDictValue>(f: unit -> 'T[]) =
 module Order =
     let orderBy (p: 'T -> 'U) =
         { new IComparer<'T> with
-            member _.Compare(x, xx) = compare (p x) (p xx)
+            member _.Compare(x, xx) = compare (p !!x) (p !!xx)
         }
 
     let orderOn p (pxOrder: IComparer<'U>) =
@@ -270,6 +242,7 @@ module Array =
     let order (eltOrder: IComparer<'T>) =
         { new IComparer<'T array> with
             member _.Compare(xs, ys) =
+                let xs,ys = nullArgCheck "xs" xs, nullArgCheck "ys" ys
                 let c = compare xs.Length ys.Length
 
                 if c <> 0 then
@@ -566,6 +539,7 @@ module List =
     let order (eltOrder: IComparer<'T>) =
         { new IComparer<'T list> with
             member _.Compare(xs, ys) =
+                let xs,ys = nullArgCheck "xs" xs, nullArgCheck "ys" ys
                 let rec loop xs ys =
                     match xs, ys with
                     | [], [] -> 0
@@ -831,13 +805,16 @@ module String =
 
     let (|StartsWith|_|) pattern value =
         if String.IsNullOrWhiteSpace value then None
-        elif value.StartsWithOrdinal pattern then Some()
+        elif (!!value).StartsWithOrdinal pattern then Some()
         else None
 
-    let (|Contains|_|) pattern value =
-        if String.IsNullOrWhiteSpace value then None
-        elif value.Contains pattern then Some()
-        else None
+    let (|Contains|_|) (pattern:string) value =
+        match value with        
+        | value when String.IsNullOrWhiteSpace value -> None
+        | null -> None
+        | value ->
+            if value.Contains pattern then Some()
+            else None
 
     let getLines (str: string) =
         use reader = new StringReader(str)
@@ -976,7 +953,11 @@ module ResultOrException =
         | Exception _err -> f ()
 
 /// Generates unique stamps
-type UniqueStampGenerator<'T when 'T: equality>() =
+type UniqueStampGenerator<'T when 'T: equality
+#if !NO_CHECKNULLS
+    and 'T:not null
+#endif
+    >() =
     let encodeTable = ConcurrentDictionary<'T, Lazy<int>>(HashIdentity.Structural)
     let mutable nItems = -1
 
@@ -988,7 +969,11 @@ type UniqueStampGenerator<'T when 'T: equality>() =
     member _.Table = encodeTable.Keys
 
 /// memoize tables (all entries cached, never collected)
-type MemoizationTable<'T, 'U>(compute: 'T -> 'U, keyComparer: IEqualityComparer<'T>, ?canMemoize) =
+type MemoizationTable<'T, 'U
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    >(compute: 'T -> 'U, keyComparer: IEqualityComparer<'T>, ?canMemoize) =
 
     let table = new ConcurrentDictionary<'T, Lazy<'U>>(keyComparer)
     let computeFunc = Func<_, _>(fun key -> lazy (compute key))
@@ -1004,7 +989,11 @@ type MemoizationTable<'T, 'U>(compute: 'T -> 'U, keyComparer: IEqualityComparer<
             compute x
 
 /// A thread-safe lookup table which is assigning an auto-increment stamp with each insert
-type internal StampedDictionary<'T, 'U>(keyComparer: IEqualityComparer<'T>) =
+type internal StampedDictionary<'T, 'U
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    >(keyComparer: IEqualityComparer<'T>) =
     let table = new ConcurrentDictionary<'T, Lazy<int * 'U>>(keyComparer)
     let mutable count = -1
 
@@ -1046,7 +1035,7 @@ type LazyWithContext<'T, 'Ctxt> =
 
         /// This field holds either the function to run or a LazyWithContextFailure object recording the exception raised
         /// from running the function. It is null if the thunk has been evaluated successfully.
-        mutable funcOrException: obj
+        mutable funcOrException: objnull
 
         /// A helper to ensure we rethrow the "original" exception
         findOriginalException: exn -> exn
@@ -1133,7 +1122,7 @@ module IPartialEqualityComparer =
     let On f (c: IPartialEqualityComparer<_>) =
         { new IPartialEqualityComparer<_> with
             member _.InEqualityRelation x = c.InEqualityRelation(f x)
-            member _.Equals(x, y) = c.Equals(f x, f y)
+            member _.Equals(x, y) = c.Equals(f !!x, f !!y)
             member _.GetHashCode x = c.GetHashCode(f x)
         }
 

src/Compiler/Utilities/illib.fsi

@@ -41,31 +41,6 @@ module internal PervasiveAutoOpens =
     /// Returns true if the list contains exactly 1 element. Otherwise false.
     val inline isSingleton: l: 'a list -> bool
 
-    /// Returns true if the argument is non-null.
-    val inline isNotNull: x: 'T -> bool when 'T: null
-
-#if NO_CHECKNULLS
-    /// Indicates that a type may be null. 'MaybeNull<string>' is used internally in the F# compiler as
-    /// replacement for 'string?' to align with FS-1060.
-    type 'T MaybeNull when 'T: null and 'T: not struct = 'T
-
-    /// Asserts the argument is non-null and raises an exception if it is
-    val inline (|NonNullQuick|): 'T MaybeNull -> 'T
-
-    /// Match on the nullness of an argument.
-    val inline (|Null|NonNull|): 'T MaybeNull -> Choice<unit, 'T>
-
-    /// Asserts the argument is non-null and raises an exception if it is
-    val inline nonNull: x: 'T MaybeNull -> 'T
-
-    /// Checks the argument is non-null
-    val inline nullArgCheck: paramName: string -> x: 'T MaybeNull -> 'T
-#else
-    /// Indicates that a type may be null. 'MaybeNull<string>' used internally in the F# compiler as unchecked
-    /// replacement for 'string?'
-    type 'T MaybeNull when 'T: not null and 'T: not struct = 'T | null
-#endif
-
     val inline (===): x: 'a -> y: 'a -> bool when 'a: not struct
 
     /// Per the docs the threshold for the Large Object Heap is 85000 bytes: https://learn.microsoft.com/dotnet/standard/garbage-collection/large-object-heap#how-an-object-ends-up-on-the-large-object-heap-and-how-gc-handles-them
@@ -111,7 +86,12 @@ type DelayInitArrayMap<'T, 'TDictKey, 'TDictValue> =
 
 module internal Order =
 
-    val orderBy: p: ('T -> 'U) -> IComparer<'T> when 'U: comparison
+    val orderBy: p: ('T -> 'U) -> IComparer<'T> 
+        when 'U: comparison
+#if !NO_CHECKNULLS
+        and 'T:not null
+        and 'T:not struct
+#endif
 
     val orderOn: p: ('T -> 'U) -> pxOrder: IComparer<'U> -> IComparer<'T>
 
@@ -403,7 +383,11 @@ module internal ResultOrException =
     val otherwise: f: (unit -> ResultOrException<'a>) -> x: ResultOrException<'a> -> ResultOrException<'a>
 
 /// Generates unique stamps
-type internal UniqueStampGenerator<'T when 'T: equality> =
+type internal UniqueStampGenerator<'T when 'T: equality
+#if !NO_CHECKNULLS
+    and 'T:not null
+#endif
+    > =
 
     new: unit -> UniqueStampGenerator<'T>
 
@@ -412,7 +396,11 @@ type internal UniqueStampGenerator<'T when 'T: equality> =
     member Table: ICollection<'T>
 
 /// Memoize tables (all entries cached, never collected unless whole table is collected)
-type internal MemoizationTable<'T, 'U> =
+type internal MemoizationTable<'T, 'U
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    > =
 
     new:
         compute: ('T -> 'U) * keyComparer: IEqualityComparer<'T> * ?canMemoize: ('T -> bool) -> MemoizationTable<'T, 'U>
@@ -420,7 +408,11 @@ type internal MemoizationTable<'T, 'U> =
     member Apply: x: 'T -> 'U
 
 /// A thread-safe lookup table which is assigning an auto-increment stamp with each insert
-type internal StampedDictionary<'T, 'U> =
+type internal StampedDictionary<'T, 'U
+#if !NO_CHECKNULLS
+    when 'T:not null
+#endif
+    > =
 
     new: keyComparer: IEqualityComparer<'T> -> StampedDictionary<'T, 'U>
 
@@ -453,7 +445,11 @@ type internal LazyWithContext<'T, 'ctxt> =
 
 /// Intern tables to save space.
 module internal Tables =
-    val memoize: f: ('a -> 'b) -> ('a -> 'b) when 'a: equality
+    val memoize: f: ('a -> 'b) -> ('a -> 'b) 
+        when 'a: equality
+#if !NO_CHECKNULLS && NET8_0_OR_GREATER
+        and 'a:not null
+#endif
 
 /// Interface that defines methods for comparing objects using partial equality relation
 type internal IPartialEqualityComparer<'T> =
@@ -463,6 +459,10 @@ type internal IPartialEqualityComparer<'T> =
 /// Interface that defines methods for comparing objects using partial equality relation
 module internal IPartialEqualityComparer =
     val On: f: ('a -> 'b) -> c: IPartialEqualityComparer<'b> -> IPartialEqualityComparer<'a>
+#if !NO_CHECKNULLS
+        when 'a:not null
+        and 'a:not struct
+#endif
 
     /// Like Seq.distinctBy but only filters out duplicates for some of the elements
     val partialDistinctBy: per: IPartialEqualityComparer<'T> -> seq: 'T list -> 'T list

src/Compiler/Utilities/sformat.fs

@@ -7,9 +7,6 @@
 // The one implementation file is used because we keep the implementations of
 // structured formatting the same for fsi.exe and '%A' printing. However F# Interactive has
 // a richer feature set.
-
-#nowarn "52" // The value has been copied to ensure the original is not mutated by this operation
-
 #if COMPILER
 namespace FSharp.Compiler.Text
 #else
@@ -17,6 +14,12 @@ namespace FSharp.Compiler.Text
 namespace Microsoft.FSharp.Text.StructuredPrintfImpl
 #endif
 
+#nowarn "52" // The value has been copied to ensure the original is not mutated by this operation
+// 3261 and 3262 Nullness warnings - this waits for LKG update, since this file is included in fsharp.core and fsharp.compiler.service and goes via proto build.
+// Supporting all possible combinations of available library+compiler versions would complicate code in this source files too much at the moment.
+#nowarn "3261"
+#nowarn "3262"
+
 // Breakable block layout implementation.
 // This is a fresh implementation of pre-existing ideas.
 
@@ -29,6 +32,9 @@ open Microsoft.FSharp.Core
 open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
 open Microsoft.FSharp.Reflection
 open Microsoft.FSharp.Collections
+#if COMPILER
+open Internal.Utilities.Library
+#endif
 
 [<StructuralEquality; NoComparison>]
 type TextTag =
@@ -417,7 +423,7 @@ type FormatOptions =
         FloatingPointFormat: string
         AttributeProcessor: string -> (string * string) list -> bool -> unit
 #if COMPILER // This is the PrintIntercepts extensibility point currently revealed by fsi.exe's AddPrinter
-        PrintIntercepts: (IEnvironment -> obj -> Layout option) list
+        PrintIntercepts: (IEnvironment -> objnull -> Layout option) list
         StringLimit: int
 #endif
         FormatProvider: IFormatProvider
@@ -1592,7 +1598,7 @@ module Display =
 
                     match text with
                     | null -> ""
-                    | _ -> text
+                    | text -> text
                 with e ->
                     // If a .ToString() call throws an exception, catch it and use the message as the result.
                     // This may be informative, e.g. division by zero etc...

src/Compiler/Utilities/sformat.fsi

@@ -22,8 +22,8 @@ open System
 open System.IO
 open Microsoft.FSharp.Core
 open Microsoft.FSharp.Collections
-
 #if COMPILER
+open Internal.Utilities.Library
 
 /// Data representing joints in structured layouts of terms.  The representation
 /// of this data type is only for the consumption of formatting engines.
@@ -366,7 +366,7 @@ type internal FormatOptions =
     { FloatingPointFormat: string
       AttributeProcessor: string -> (string * string) list -> bool -> unit
 #if COMPILER  // FSharp.Core.dll: PrintIntercepts aren't used there
-      PrintIntercepts: (IEnvironment -> obj -> Layout option) list
+      PrintIntercepts: (IEnvironment -> objnull -> Layout option) list
       StringLimit: int
 #endif
       FormatProvider: IFormatProvider

src/Compiler/Utilities/sr.fs

@@ -6,6 +6,7 @@ open System
 open Microsoft.FSharp.Core
 open Microsoft.FSharp.Collections
 open Microsoft.FSharp.Reflection
+open Internal.Utilities.Library
 
 module internal SR =
     let private resources =
@@ -20,7 +21,7 @@ module internal SR =
         if isNull s then
             System.Diagnostics.Debug.Assert(false, sprintf "**RESOURCE ERROR**: Resource token %s does not exist!" name)
 #endif
-        s
+        !!s
 
 module internal DiagnosticMessage =
 
@@ -53,7 +54,7 @@ module internal DiagnosticMessage =
         // PERF: this technique is a bit slow (e.g. in simple cases, like 'sprintf "%x"')
         mkFunctionValue tys (fun inp -> impl rty inp)
 
-    let capture1 (fmt: string) i args ty (go: obj list -> System.Type -> int -> obj) : obj =
+    let capture1 (fmt: string) i args ty (go: objnull list -> System.Type -> int -> obj) : obj =
         match fmt[i] with
         | '%' -> go args ty (i + 1)
         | 'd'
@@ -75,7 +76,7 @@ module internal DiagnosticMessage =
             if i >= len || (fmt[i] = '%' && i + 1 >= len) then
                 let b = System.Text.StringBuilder()
                 b.AppendFormat(messageString, (Array.ofList (List.rev args))) |> ignore
-                box (b.ToString())
+                !!(box (b.ToString()))
             // REVIEW: For these purposes, this should be a nop, but I'm leaving it
             // in case we ever decide to support labels for the error format string
             // E.g., "<name>%s<foo>%d"
@@ -99,7 +100,7 @@ module internal DiagnosticMessage =
         // validate that the message string exists
         let fmtString = fmt.Value
 
-        if isNull messageString then
+        if isNull (box messageString) then
             System.Diagnostics.Debug.Assert(false, sprintf "**DECLARED MESSAGE ERROR** String resource %s does not exist" messageID)
             messageString <- ""
 
@@ -149,7 +150,7 @@ module internal DiagnosticMessage =
 
                 nFmt
 
-        let nHoles, holes = countFormatHoles messageString
+        let nHoles, holes = countFormatHoles !!messageString
         let nPlaceholders = countFormatPlaceholders fmtString
 
         // first, verify that the number of holes in the message string does not exceed the
@@ -172,5 +173,5 @@ module internal DiagnosticMessage =
             )
 
 #endif
-        messageString <- postProcessString messageString
-        new ResourceString<'T>(messageString, fmt)
+        messageString <- postProcessString !!messageString
+        new ResourceString<'T>(!!messageString, fmt)

src/Compiler/pars.fsy

@@ -3,6 +3,7 @@
 %{
 
 #nowarn "1182"  // generated code has lots of unused "parseState"
+#nowarn "3261"  // the generated code would need to properly annotate nulls, e.g. changing System.Object to `obj|null`
 
 open System
 
@@ -21,6 +22,7 @@ open FSharp.Compiler.Syntax
 open FSharp.Compiler.SyntaxTrivia
 open FSharp.Compiler.Syntax.PrettyNaming
 open FSharp.Compiler.SyntaxTreeOps
+open FSharp.Compiler.Text
 open FSharp.Compiler.Text.Position
 open FSharp.Compiler.Text.Range
 open FSharp.Compiler.Xml

src/Compiler/pppars.fsy

@@ -3,6 +3,8 @@
 %{
 open FSharp.Compiler.DiagnosticsLogger
 
+#nowarn "3261"  // the generated code would need to properly annotate nulls, e.g. changing System.Object to `obj|null`
+
 let dummy       = IfdefId("DUMMY")
 
 let doNothing _ dflt=

src/FSharp.Build/FSharp.Build.fsproj

@@ -4,11 +4,12 @@
 
   <PropertyGroup>
     <OutputType>Library</OutputType>
-    <TargetFramework Condition="'$(Configuration)' != 'Proto'">netstandard2.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <TargetFrameworks Condition="'$(Configuration)' == 'Proto'">netstandard2.0</TargetFrameworks>
     <AssemblyName>FSharp.Build</AssemblyName>
     <NoWarn>$(NoWarn);75</NoWarn> <!-- InternalCommandLineOption -->
     <AllowCrossTargeting>true</AllowCrossTargeting>
+    <CheckNulls>true</CheckNulls>
     <DefineConstants>$(DefineConstants);LOCALIZATION_FSBUILD</DefineConstants>
     <NoWarn>$(NoWarn);NU1701;FS0075</NoWarn>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
@@ -30,6 +31,7 @@
 
   <ItemGroup>
     <InternalsVisibleTo Include="VisualFSharp.UnitTests" />
+    <Compile Include="..\Compiler\Utilities\NullnessShims.fs" />
     <EmbeddedText Include="FSBuild.txt" />
     <EmbeddedText Include="..\Compiler\Facilities\UtilsStrings.txt" />
     <Compile Include="..\Compiler\Facilities\CompilerLocation.fs" />

src/FSharp.Build/FSharpEmbedResourceText.fs

@@ -288,7 +288,8 @@ open Printf
         if isNull s then
             System.Diagnostics.Debug.Assert(false, sprintf ""**RESOURCE ERROR**: Resource token %s does not exist!"" name)
     #endif
-        s
+        Unchecked.nonNull s
+
 
     static let mkFunctionValue (tys: System.Type[]) (impl:obj->obj) =
         FSharpValue.MakeFunction(FSharpType.MakeFunctionType(tys.[0],tys.[1]), impl)
@@ -313,7 +314,7 @@ open Printf
         // PERF: this technique is a bit slow (e.g. in simple cases, like 'sprintf ""%x""')
         mkFunctionValue tys (fun inp -> impl rty inp)
 
-    static let capture1 (fmt:string) i args ty (go: obj list -> System.Type -> int -> obj) : obj =
+    static let capture1 (fmt:string) i args ty (go: objnull list -> System.Type -> int -> obj) : obj =
         match fmt.[i] with
         | '%' -> go args ty (i+1)
         | 'd'
@@ -335,7 +336,7 @@ open Printf
             if i >= len ||  (fmt.[i] = '%' && i+1 >= len) then
                 let b = new System.Text.StringBuilder()
                 b.AppendFormat(messageString, [| for x in List.rev args -> x |]) |> ignore
-                box(b.ToString())
+                box(b.ToString()) |> Unchecked.nonNull
             // REVIEW: For these purposes, this should be a nop, but I'm leaving it
             // in incase we ever decide to support labels for the error format string
             // E.g., ""<name>%s<foo>%d""

src/FSharp.DependencyManager.Nuget/FSharp.DependencyManager.Nuget.fsproj

@@ -32,6 +32,7 @@
   </Target>
 
   <ItemGroup>
+    <Compile Include="$(FSharpSourcesRoot)\Compiler\Utilities\NullnessShims.fs" />
     <EmbeddedText Include="FSDependencyManager.txt" />
     <EmbeddedText Include="$(FSharpSourcesRoot)\Compiler\Facilities\UtilsStrings.txt" />
     <Compile Include="$(FSharpSourcesRoot)\Compiler\Facilities\CompilerLocation.fsi" />

tests/FSharp.Compiler.ComponentTests/CompilerService/AsyncMemoize.fs

@@ -36,8 +36,11 @@ let rec internal spinFor (duration: TimeSpan) =
             return! spinFor remaining
     }
 
-
+#if BUILDING_WITH_LKG
 type internal EventRecorder<'a, 'b, 'c when 'a : equality and 'b : equality>(memoize: AsyncMemoize<'a,'b,'c>) as self =
+#else
+type internal EventRecorder<'a, 'b, 'c when 'a : equality and 'b : equality and 'a:not null and 'b:not null>(memoize: AsyncMemoize<'a,'b,'c>) as self =
+#endif
 
     let events = ConcurrentQueue()
 

tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj

@@ -85,7 +85,10 @@
     <PackageReference Include="Dotnet.ProjInfo" Version="0.37.0" />
     <!-- Force a newer Newtonsoft.Json version to avoid conflicts. -->
     <PackageReference Include="Newtonsoft.Json" Version="$(NewtonsoftJsonVersion)" />
-    <ProjectReference Include="..\..\src\Compiler\FSharp.Compiler.Service.fsproj" />
+	<ProjectReference Include="..\..\src\Compiler\FSharp.Compiler.Service.fsproj">
+		<!-- If tests would use net9.0, the surface area changes because of different set of Assembly references -->
+		<SetTargetFramework>TargetFramework=netstandard2.0</SetTargetFramework>
+	</ProjectReference>
     <ProjectReference Include="..\..\tests\FSharp.Test.Utilities\FSharp.Test.Utilities.fsproj" />
     <ProjectReference Include="..\service\data\CSharp_Analysis\CSharp_Analysis.csproj" />
   </ItemGroup>

tests/fsharp/Compiler/Libraries/Core/Unchecked/DefaultOfTests.fs

@@ -33,7 +33,7 @@ module ``DefaultOf Tests`` =
 
     [<Test>]
     let `` Unchecked defaultof reference types``() =
-        Assert.areEqual Unchecked.defaultof<ClassType> null
+        Assert.areEqual Unchecked.defaultof<ClassType|null> null
         Assert.areEqual (box Unchecked.defaultof<DUType>) null
         Assert.areEqual (box Unchecked.defaultof<RecordType>) null
         Assert.areEqual (box Unchecked.defaultof<InterfaceType>) null

vsintegration/src/FSharp.VS.FSI/FSharp.VS.FSI.fsproj

@@ -18,6 +18,7 @@
   </ItemGroup>
 
   <ItemGroup>
+	<Compile Include="$(FSharpSourcesRoot)\Compiler\Utilities\NullnessShims.fs"/>
     <EmbeddedText Include="VFSIstrings.txt" />
     <Compile Include="AssemblyInfo.fs" />
     <Compile Include="FSharp.VS.FSI.Attributes.fs" />

vsintegration/tests/Salsa/VisualFSharp.Salsa.fsproj

@@ -15,6 +15,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Include="$(FSharpSourcesRoot)\Compiler\Utilities\NullnessShims.fs"/>
     <EmbeddedText Include="$(FSharpSourcesRoot)\Compiler\Facilities\UtilsStrings.txt" />
     <Compile Include="$(FSharpSourcesRoot)\Compiler\Facilities\CompilerLocation.fs">
       <Link>CompilerLocation.fs</Link>

vsintegration/tests/UnitTests/VisualFSharp.UnitTests.fsproj

@@ -16,6 +16,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="$(FSharpSourcesRoot)\Compiler\Utilities\NullnessShims.fs"/>
     <Compile Include="AssemblyResolver.fs" />
     <Compile Include="$(FSharpSourcesRoot)\Compiler\Utilities\InternalCollections.fsi">
       <Link>Internal.Utilities.Collections.fsi</Link>