diff --git a/src/Compiler/Checking/NicePrint.fs b/src/Compiler/Checking/NicePrint.fs
index 7f0779c062..da4caaf7f2 100644
--- a/src/Compiler/Checking/NicePrint.fs
+++ b/src/Compiler/Checking/NicePrint.fs
@@ -708,7 +708,15 @@ module PrintTypes =
         match Zmap.tryFind typar env.inplaceConstraints with
         | Some typarConstraintTy ->
             if Zset.contains typar env.singletons then
-                leftL (tagPunctuation "#") ^^ layoutTypeWithInfo denv env typarConstraintTy
+                let tyLayout =
+                    match typarConstraintTy with
+                    | TType_app (tyconRef = tc; typeInstantiation = ti)
+                        when ti.Length > 0 && not (usePrefix denv tc) ->
+                        layoutTypeWithInfo denv env typarConstraintTy
+                        |> bracketL
+                    | _ -> layoutTypeWithInfo denv env typarConstraintTy
+
+                leftL (tagPunctuation "#") ^^ tyLayout
             else
                 (varL ^^ sepL (tagPunctuation ":>") ^^ layoutTypeWithInfo denv env typarConstraintTy) |> bracketL
 
diff --git a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
index 6c5c5f76c2..93ff861fe6 100644
--- a/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
+++ b/tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj
@@ -251,6 +251,7 @@
     <Compile Include="Signatures\SigGenerationRoundTripTests.fs" />
     <Compile Include="Signatures\NestedTypeTests.fs" />
     <Compile Include="Signatures\MissingDiagnostic.fs" />
+    <Compile Include="Signatures\HashConstraintTests.fs" />
     <Compile Include="StaticLinking\StaticLinking.fs" />
     <Compile Include="FSharpChecker\CommonWorkflows.fs" />
     <Compile Include="FSharpChecker\SymbolUse.fs" />
diff --git a/tests/FSharp.Compiler.ComponentTests/Signatures/ArrayTests.fs b/tests/FSharp.Compiler.ComponentTests/Signatures/ArrayTests.fs
index 7d74903c4e..74a69d7e70 100644
--- a/tests/FSharp.Compiler.ComponentTests/Signatures/ArrayTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Signatures/ArrayTests.fs
@@ -1,4 +1,4 @@
-module Signatures.SignatureTests
+module Signatures.ArrayTests
 
 open Xunit
 open Signatures.TestHelpers
diff --git a/tests/FSharp.Compiler.ComponentTests/Signatures/HashConstraintTests.fs b/tests/FSharp.Compiler.ComponentTests/Signatures/HashConstraintTests.fs
new file mode 100644
index 0000000000..beb33a8bac
--- /dev/null
+++ b/tests/FSharp.Compiler.ComponentTests/Signatures/HashConstraintTests.fs
@@ -0,0 +1,33 @@
+module Signatures.HashConstraintTests
+
+open Xunit
+open FsUnit
+open FSharp.Test.Compiler
+open Signatures.TestHelpers
+
+[<Fact>]
+let ``Optional hash constraint`` () =
+    FSharp
+        """
+module Foo
+
+[<Interface>]
+type Node =
+    abstract Children: int array
+
+let noa<'n when 'n :> Node> (n: 'n option) =
+    match n with
+    | None -> Array.empty
+    | Some n -> [| n :> Node |]
+"""
+    |> printSignatures
+    |> should
+        equal
+        """
+module Foo
+
+type Node =
+
+  abstract Children: int array
+
+val noa: n: #Node option -> Node array"""
diff --git a/tests/FSharp.Compiler.ComponentTests/Signatures/MissingDiagnostic.fs b/tests/FSharp.Compiler.ComponentTests/Signatures/MissingDiagnostic.fs
index 2d263fa728..55181649b0 100644
--- a/tests/FSharp.Compiler.ComponentTests/Signatures/MissingDiagnostic.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Signatures/MissingDiagnostic.fs
@@ -1,4 +1,4 @@
-module FSharp.Compiler.ComponentTests.Signatures.MissingDiagnostic
+module Signatures.MissingDiagnostic
 
 open Xunit
 open FSharp.Test
diff --git a/tests/FSharp.Compiler.ComponentTests/Signatures/SigGenerationRoundTripTests.fs b/tests/FSharp.Compiler.ComponentTests/Signatures/SigGenerationRoundTripTests.fs
index 469fa879b4..b0ff8e995c 100644
--- a/tests/FSharp.Compiler.ComponentTests/Signatures/SigGenerationRoundTripTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/Signatures/SigGenerationRoundTripTests.fs
@@ -28,7 +28,7 @@ let ``Generate and compile`` implFileName =
     |> withLangVersionPreview
     |> withDefines ["TESTS_AS_APP";"COMPILED"]
     |> ignoreWarnings
+    |> withOptions [ "--warnaserror:64" ]
     |> asExe
     |> compile
     |> shouldSucceed
-
diff --git a/tests/FSharp.Compiler.ComponentTests/Signatures/TestCasesForGenerationRoundTrip/hash_constraint_seq.fsx b/tests/FSharp.Compiler.ComponentTests/Signatures/TestCasesForGenerationRoundTrip/hash_constraint_seq.fsx
new file mode 100644
index 0000000000..60bdb225ad
--- /dev/null
+++ b/tests/FSharp.Compiler.ComponentTests/Signatures/TestCasesForGenerationRoundTrip/hash_constraint_seq.fsx
@@ -0,0 +1,7 @@
+module Foo
+
+open System
+open System.Threading.Tasks
+
+let mapWithAdditionalDependenies
+    (mapping: 'a -> 'b * #seq<#IDisposable>) = 0