Fix #3542: Invalid explicit cast for implicit conversion to generic struct with interface type constraint

Author: siegfriedpammer

ICSharpCode.Decompiler.Tests/Semantics/ConversionTests.cs

@@ -529,6 +529,17 @@ public void UserDefinedImplicitConversion()
 			Assert.That(c.Method.FullName, Is.EqualTo("System.DateTimeOffset.op_Implicit"));
 
 			Assert.That(ImplicitConversion(typeof(DateTimeOffset), typeof(DateTime)), Is.EqualTo(C.None));
+
+			ITypeDefinition classImplementingIDisposable = compilation.FindType(typeof(ClassImplementingIDisposable)).GetDefinition();
+			ITypeDefinition genericStructWithIDisposableConstraintAndImplicitConversion = compilation.FindType(typeof(GenericStructWithIDisposableConstraintAndImplicitConversion<>)).GetDefinition();
+			IType genericStructIDisposableInstance = new ParameterizedType(genericStructWithIDisposableConstraintAndImplicitConversion, ImmutableArray.Create(compilation.FindType(typeof(IDisposable))));
+
+			// C => S<I>
+			Conversion c2 = conversions.ImplicitConversion(classImplementingIDisposable, genericStructIDisposableInstance);
+			Assert.That(c2.IsImplicit && c2.IsUserDefined);
+			Assert.That(c2.Method.FullName, Is.EqualTo("ICSharpCode.Decompiler.Tests.TypeSystem.GenericStructWithIDisposableConstraintAndImplicitConversion.op_Implicit"));
+
+			Assert.That(conversions.ImplicitConversion(genericStructIDisposableInstance, classImplementingIDisposable), Is.EqualTo(C.None));
 		}
 
 		[Test]

ICSharpCode.Decompiler.Tests/TypeSystem/TypeSystemTestCase.cs

@@ -559,6 +559,19 @@ class Inner { }
 		}
 	}
 
+	public struct GenericStructWithIDisposableConstraintAndImplicitConversion<T> where T : IDisposable
+	{
+		public static implicit operator GenericStructWithIDisposableConstraintAndImplicitConversion<T>(T s)
+		{
+			return default(GenericStructWithIDisposableConstraintAndImplicitConversion<T>);
+		}
+	}
+
+	public class ClassImplementingIDisposable : IDisposable
+	{
+		public void Dispose() { }
+	}
+
 	public class ClassWithAttributeOnTypeParameter<[Double(2)] T> { }
 
 	[Guid("790C6E0B-9194-4cc9-9426-A48A63185696"), InterfaceType(ComInterfaceType.InterfaceIsDual)]

ICSharpCode.Decompiler/CSharp/Resolver/CSharpConversions.cs

@@ -955,13 +955,12 @@ bool IsIntegerType(IType type)
 		/// </summary>
 		bool IsEncompassedBy(IType a, IType b)
 		{
-			return a.Kind != TypeKind.Interface && b.Kind != TypeKind.Interface && StandardImplicitConversion(a, b).IsValid;
+			return StandardImplicitConversion(a, b).IsValid;
 		}
 
 		bool IsEncompassingOrEncompassedBy(IType a, IType b)
 		{
-			return a.Kind != TypeKind.Interface && b.Kind != TypeKind.Interface
-				&& (StandardImplicitConversion(a, b).IsValid || StandardImplicitConversion(b, a).IsValid);
+			return (StandardImplicitConversion(a, b).IsValid || StandardImplicitConversion(b, a).IsValid);
 		}
 
 		IType FindMostEncompassedType(IEnumerable<IType> candidates)
@@ -1027,6 +1026,13 @@ Conversion SelectOperator(IType mostSpecificSource, IType mostSpecificTarget, IL
 		Conversion UserDefinedImplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
 		{
 			// C# 4.0 spec §6.4.4 User-defined implicit conversions
+
+			// user-defined conversions are not supported with interfaces
+			if (fromType.Kind == TypeKind.Interface || toType.Kind == TypeKind.Interface)
+			{
+				return Conversion.None;
+			}
+
 			var operators = GetApplicableConversionOperators(fromResult, fromType, toType, false);
 
 			if (operators.Count > 0)
@@ -1069,6 +1075,13 @@ Conversion UserDefinedImplicitConversion(ResolveResult fromResult, IType fromTyp
 		Conversion UserDefinedExplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
 		{
 			// C# 4.0 spec §6.4.5 User-defined explicit conversions
+
+			// user-defined conversions are not supported with interfaces
+			if (fromType.Kind == TypeKind.Interface || toType.Kind == TypeKind.Interface)
+			{
+				return Conversion.None;
+			}
+
 			var operators = GetApplicableConversionOperators(fromResult, fromType, toType, true);
 			if (operators.Count > 0)
 			{