[registrar] Report a warning when the registrar export an abstract INativeObject type to Objective-C. (#6659)

* [registrar] Report a warning when the registrar export an abstract INativeObject type to Objective-C.

Exporting abstract types to Objective-C can lead to problems when at runtime
we're asked to create an instance of such a type (which we can't), so warn
when this happens.

This would have caught #6655, and the problems explained in #4969 as well.

Since this may trigger for code that's currently working fine, I'm making it a
warning instead of an error (which means adding some extra code to be able to
easily report warnings from the generator code).

* Don't assume a TypeReference can be successfully resolved every time.

docs/website/mmp-errors.md

@@ -583,6 +583,22 @@ complicated to get it right when doing it manually.
 
 If this is not the case, please file a [bug report](https://github.com/xamarin/xamarin-macios/issues/new) with a test case.
 
+### MM4178: The registrar found the abstract type '{type}' in the signature for '{member}'. Abstract types should not be used in the signature for a member exported to Objective-C.
+
+This is a warning, indicating a potential problem where a method or property
+has a parameter or return type which is abstract. The potential problem occurs
+at runtime, when the Xamarin.Mac runtime may need to create an instance of
+such a type, which will fail if the type is abstract.
+
+Possible solutions:
+
+* Modify the signature in question to not use an abstract type.
+* Make the type not abstract.
+
+If this is an API exposed by Xamarin, please file a new issue on
+[github](https://github.com/xamarin/xamarin-macios/issues/new), if it's a
+third-party binding, please contact the vendor.
+
 ## MM5xxx: GCC and toolchain
 
 ### MM51xx: compilation

docs/website/mtouch-errors.md

@@ -2545,6 +2545,22 @@ If this is not the case, please file a [bug report](https://github.com/xamarin/x
 
 The name of an Objective-C protocol can't contain certain characters which means that the `Adopts` attribute on the corresponding managed class can't have the `ProtocolType` parameter containing them. Please refer to the provided error message and fix accordingly.
 
+### MT4178: The registrar found the abstract type '{type}' in the signature for '{member}'. Abstract types should not be used in the signature for a member exported to Objective-C.
+
+This is a warning, indicating a potential problem where a method or property
+has a parameter or return type which is abstract. The potential problem occurs
+at runtime, when the Xamarin.iOS runtime may need to create an instance of
+such a type, which will fail if the type is abstract.
+
+Possible solutions:
+
+* Modify the signature in question to not use an abstract type.
+* Make the type not abstract.
+
+If this is an API exposed by Xamarin, please file a new issue on
+[github](https://github.com/xamarin/xamarin-macios/issues/new), if it's a
+third-party binding, please contact the vendor.
+
 # MT5xxx: GCC and toolchain error messages
 
 ### MT51xx: Compilation

src/ObjCRuntime/DynamicRegistrar.cs

@@ -524,18 +524,22 @@ namespace Registrar {
 			return false;
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, MethodBase method, string message, params object[] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, MethodBase method, string message, params object[] args)
 		{
 			// There doesn't seem to be a way to find the source code location
 			// for the method using System.Reflection.
-			return ErrorHelper.CreateError (code, innerException, message, args);
+			if (error)
+				return ErrorHelper.CreateError (code, innerException, message, args);
+			return ErrorHelper.CreateWarning (code, innerException, message, args);
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, Type type, string message, params object [] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, Type type, string message, params object [] args)
 		{
 			// There doesn't seem to be a way to find the source code location
 			// for the method using System.Reflection.
-			return ErrorHelper.CreateError (code, innerException, message, args);
+			if (error)
+				return ErrorHelper.CreateError (code, innerException, message, args);
+			return ErrorHelper.CreateWarning (code, innerException, message, args);
 		}
 
 		protected override string GetAssemblyQualifiedName (Type type)
@@ -635,6 +639,11 @@ namespace Registrar {
 			return type.IsInterface;
 		}
 
+		protected override bool IsAbstract (Type type)
+		{
+			return type.IsAbstract;
+		}
+
 		protected override bool IsINativeObject (Type type)
 		{
 			return typeof (INativeObject).IsAssignableFrom (type);

src/ObjCRuntime/ErrorHelper.cs

@@ -278,6 +278,11 @@ namespace ObjCRuntime {
 			return new ProductException (code, false, message, args);
 		}
 
+		public static ProductException CreateWarning (int code, Exception innerException, string message, params object [] args)
+		{
+			return new ProductException (code, false, innerException, message, args);
+		}
+
 		public static void Error (int code, Exception innerException, string message, params object[] args)
 		{
 			throw new ProductException (code, true, innerException, message, args);

src/ObjCRuntime/Registrar.cs

@@ -1110,6 +1110,7 @@ namespace Registrar {
 		protected abstract bool IsGenericType (TType type);
 		protected abstract bool IsGenericMethod (TMethod method);
 		protected abstract bool IsInterface (TType type);
+		protected abstract bool IsAbstract (TType type);
 		protected abstract TType GetGenericTypeDefinition (TType type);
 		protected abstract bool VerifyIsConstrainedToNSObject (TType type, out TType constrained_type);
 		protected abstract TType GetEnumUnderlyingType (TType type);
@@ -1120,8 +1121,8 @@ namespace Registrar {
 		protected abstract bool IsSimulatorOrDesktop { get; }
 		protected abstract bool Is64Bits { get; }
 		protected abstract bool IsDualBuildImpl { get; }
-		protected abstract Exception CreateException (int code, Exception innerException, TMethod method, string message, params object[] args);
-		protected abstract Exception CreateException (int code, Exception innerException, TType type, string message, params object [] args);
+		protected abstract Exception CreateExceptionImpl (int code, bool error, Exception innerException, TMethod method, string message, params object[] args);
+		protected abstract Exception CreateExceptionImpl (int code, bool error, Exception innerException, TType type, string message, params object [] args);
 		protected abstract string PlatformName { get; }
 		public abstract TType FindType (TType relative, string @namespace, string name);
 		protected abstract IEnumerable<TMethod> FindMethods (TType type, string name); // will return null if nothing was found
@@ -1343,43 +1344,78 @@ namespace Registrar {
 
 		protected Exception CreateException (int code, string message, params object[] args)
 		{
-			return CreateException (code, (TMethod)null, message, args);
+			return CreateExceptionImpl (code, true, message, args);
 		}
 
 		protected Exception CreateException (int code, TMethod method, string message, params object[] args)
 		{
-			return CreateException (code, null, method, message, args);
+			return CreateExceptionImpl (code, true, method, message, args);
 		}
 
 		protected Exception CreateException (int code, TProperty property, string message, params object[] args)
 		{
-			return CreateException (code, null, property, message, args);
+			return CreateExceptionImpl (code, true, property, message, args);
 		}
 
 		protected Exception CreateException (int code, TType type, string message, params object [] args)
 		{
-			return CreateException (code, null, type, message, args);
+			return CreateExceptionImpl (code, true, type, message, args);
 		}
 
 		protected Exception CreateException (int code, Exception innerException, TProperty property, string message, params object[] args)
 		{
-			if (property == null)
-				return CreateException (code, innerException, (TMethod) null, message, args);
-			var getter = GetGetMethod (property);
-			if (getter != null)
-				return CreateException (code, innerException, getter, message, args);
-			return CreateException (code, innerException, GetSetMethod (property), message, args);
+			return CreateExceptionImpl (code, true, innerException, property, message, args);
 		}
 
-		Exception CreateException (int code, ObjCMember member, string message, params object[] args)
+		Exception CreateExceptionImpl (int code, bool error, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, error, (TMethod) null, message, args);
+		}
+
+		Exception CreateExceptionImpl (int code, bool error, TMethod method, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, error, null, method, message, args);
+		}
+
+		Exception CreateExceptionImpl (int code, bool error, TProperty property, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, error, null, property, message, args);
+		}
+
+		Exception CreateExceptionImpl (int code, bool error, TType type, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, error, null, type, message, args);
+		}
+
+		Exception CreateExceptionImpl (int code, bool error, Exception innerException, TProperty property, string message, params object [] args)
+		{
+			if (property == null)
+				return CreateExceptionImpl (code, error, innerException, (TMethod) null, message, args);
+			var getter = GetGetMethod (property);
+			if (getter != null)
+				return CreateExceptionImpl (code, error, innerException, getter, message, args);
+			return CreateExceptionImpl (code, error, innerException, GetSetMethod (property), message, args);
+		}
+
+		Exception CreateException (int code, ObjCMember member, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, true, member, message, args);
+		}
+
+		Exception CreateExceptionImpl (int code, bool error, ObjCMember member, string message, params object[] args)
 		{
 			var method = member as ObjCMethod;
 			if (method != null)
-				return CreateException (code, method.Method, message, args);
+				return CreateExceptionImpl (code, error, method.Method, message, args);
 			var property = member as ObjCProperty;
 			if (property != null)
-				return CreateException (code, property.Property, message, args);
-			return CreateException (code, message, args);
+				return CreateExceptionImpl (code, error, property.Property, message, args);
+			return CreateExceptionImpl (code, error, message, args);
+		}
+
+		Exception CreateWarning (int code, ObjCMember member, string message, params object [] args)
+		{
+			return CreateExceptionImpl (code, false, member, message, args);
 		}
 
 		protected string GetDescriptiveMethodName (TMethod method)
@@ -2614,6 +2650,8 @@ namespace Registrar {
 				return "#";
 
 			if (IsINativeObject (type)) {
+				if (!IsGenericType (type) && !IsInterface (type) && !IsNSObject (type) && IsAbstract (type))
+					ErrorHelper.Show (CreateWarning (4178, member, $"The registrar found the abstract type '{type.FullName}' in the signature for '{member.FullName}'. Abstract types should not be used in the signature for a member exported to Objective-C."));
 				if (IsNSObject (type) && forProperty) {
 					return "@\"" + GetExportedTypeName (type) + "\"";
 				} else {

tests/mtouch/RegistrarTest.cs

@@ -1794,6 +1794,55 @@ class CTP4 : CTP3 {
 				}
 			}
 		}
+
+		[Test]
+		public void MT4178 ()
+		{
+			using (var mtouch = new MTouchTool ()) {
+				var code = @"
+using System;
+
+using Foundation;
+using ObjCRuntime;
+
+abstract class SomeNativeObject : INativeObject
+{
+	public IntPtr Handle { get; set; }
+	SomeNativeObject (IntPtr handle, bool owns) { }
+}
+
+class C : NSObject {
+	[Export (""M1:"")]
+	public void M1 (SomeNativeObject obj)
+	{
+	}
+
+	[Export (""M2"")]
+	public SomeNativeObject M2 ()
+	{
+		return null;
+	}
+
+	[Export (""M3"")]
+	public SomeNativeObject M3 {
+		get;
+		set;
+	}
+	static void Main () {}
+}
+";
+
+				mtouch.CreateTemporaryApp (code: code, extraArg: "-debug");
+				mtouch.Registrar = MTouchRegistrar.Static;
+				mtouch.Linker = MTouchLinker.DontLink;
+				mtouch.AssertExecute (MTouchAction.BuildSim, "build");
+				mtouch.AssertWarningCount (4);
+				mtouch.AssertWarning (4178, $"The registrar found the abstract type 'SomeNativeObject' in the signature for 'C.get_M3'. Abstract types should not be used in the signature for a member exported to Objective-C.", "testApp.cs", 27);
+				mtouch.AssertWarning (4178, $"The registrar found the abstract type 'SomeNativeObject' in the signature for 'C.set_M3'. Abstract types should not be used in the signature for a member exported to Objective-C.", "testApp.cs", 28);
+				mtouch.AssertWarning (4178, $"The registrar found the abstract type 'SomeNativeObject' in the signature for 'C.M1'. Abstract types should not be used in the signature for a member exported to Objective-C.", "testApp.cs", 16);
+				mtouch.AssertWarning (4178, $"The registrar found the abstract type 'SomeNativeObject' in the signature for 'C.M2'. Abstract types should not be used in the signature for a member exported to Objective-C.", "testApp.cs", 21);
+			}
+		}
 	}
 }
 

tools/common/ClassicStaticRegistrar.cs

@@ -616,14 +616,14 @@ namespace Registrar {
 			}
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, MethodDefinition method, string message, params object[] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, MethodDefinition method, string message, params object[] args)
 		{
-			return ErrorHelper.CreateError (App, code, innerException, method, message, args);
+			return ErrorHelper.Create (App, code, error, innerException, method, message, args);
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, TypeReference type, string message, params object [] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, TypeReference type, string message, params object [] args)
 		{
-			return ErrorHelper.CreateError (App, code, innerException, type, message, args);
+			return ErrorHelper.Create (App, code, error, innerException, type, message, args);
 		}
 
 		protected override bool ContainsPlatformReference (AssemblyDefinition assembly)
@@ -907,6 +907,11 @@ namespace Registrar {
 			return type.Resolve ().IsInterface;
 		}
 
+		protected override bool IsAbstract (TypeReference type)
+		{
+			return type.Resolve ().IsAbstract;
+		}
+
 		protected override TypeReference[] GetInterfaces (TypeReference type)
 		{
 			var td = type.Resolve ();

tools/common/StaticRegistrar.cs

@@ -805,16 +805,16 @@ namespace Registrar {
 			}
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, MethodDefinition method, string message, params object[] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, MethodDefinition method, string message, params object[] args)
 		{
-			return ErrorHelper.CreateError (App, code, innerException, method, message, args);
+			return ErrorHelper.Create (App, code, error, innerException, method, message, args);
 		}
 
-		protected override Exception CreateException (int code, Exception innerException, TypeReference type, string message, params object [] args)
+		protected override Exception CreateExceptionImpl (int code, bool error, Exception innerException, TypeReference type, string message, params object [] args)
 		{
-			return ErrorHelper.CreateError (App, code, innerException, type, message, args);
+			return ErrorHelper.Create (App, code, error, innerException, type, message, args);
 		}
-
+		
 		protected override bool ContainsPlatformReference (AssemblyDefinition assembly)
 		{
 			if (assembly.Name.Name == PlatformAssembly)
@@ -1099,7 +1099,16 @@ namespace Registrar {
 
 		protected override bool IsInterface (TypeReference type)
 		{
-			return type.Resolve ().IsInterface;
+			if (type.IsArray)
+				return false;
+			return type.Resolve ()?.IsInterface == true;
+		}
+
+		protected override bool IsAbstract (TypeReference type)
+		{
+			if (type.IsArray)
+				return false;
+			return type.Resolve ()?.IsAbstract == true;
 		}
 
 		protected override TypeReference[] GetInterfaces (TypeReference type)