Ensure stack types for call arguments are correct.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ExpressionTrees.cs

@@ -218,7 +218,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		public void MembersBuiltin()
 		{
 			ExpressionTrees.ToCode(ExpressionTrees.X(), () => 1.23m.ToString());
-			ExpressionTrees.ToCode(ExpressionTrees.X(), () => AttributeTargets.All.HasFlag((Enum)AttributeTargets.Assembly));
+			ExpressionTrees.ToCode(ExpressionTrees.X(), () => ((Enum)(object)AttributeTargets.All).HasFlag((Enum)AttributeTargets.Assembly));
 			ExpressionTrees.ToCode(ExpressionTrees.X(), () => "abc".Length == 3);
 			ExpressionTrees.ToCode(ExpressionTrees.X(), () => 'a'.CompareTo('b') < 0);
 		}

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -1130,15 +1130,18 @@ namespace ICSharpCode.Decompiler.IL
 			return new StObj(target, value, type);
 		}
 
+		IType constrainedPrefix;
+
 		private ILInstruction DecodeConstrainedCall()
 		{
-			var typeRef = ReadAndDecodeTypeReference();
+			constrainedPrefix = ReadAndDecodeTypeReference();
 			var inst = DecodeInstruction();
 			var call = UnpackPush(inst) as CallInstruction;
 			if (call != null)
-				call.ConstrainedTo = typeRef;
+				Debug.Assert(call.ConstrainedTo == constrainedPrefix);
 			else
 				Warn("Ignored invalid 'constrained' prefix");
+			constrainedPrefix = null;
 			return inst;
 		}
 
@@ -1196,7 +1199,7 @@ namespace ICSharpCode.Decompiler.IL
 				arguments[firstArgument + i] = Pop(method.Parameters[i].Type.GetStackType());
 			}
 			if (firstArgument == 1) {
-				arguments[0] = Pop();
+				arguments[0] = Pop(CallInstruction.ExpectedTypeForThisPointer(constrainedPrefix ?? method.DeclaringType));
 			}
 			switch (method.DeclaringType.Kind) {
 				case TypeKind.Array:
@@ -1224,6 +1227,7 @@ namespace ICSharpCode.Decompiler.IL
 				default:
 					var call = CallInstruction.Create(opCode, method);
 					call.ILStackWasEmpty = currentStack.IsEmpty;
+					call.ConstrainedTo = constrainedPrefix;
 					call.Arguments.AddRange(arguments);
 					if (call.ResultType != StackType.Void)
 						return Push(call);

ICSharpCode.Decompiler/IL/Instructions/CallInstruction.cs

@@ -74,6 +74,33 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
+		/// <summary>
+		/// Gets the expected stack type for passing the this pointer in a method call.
+		/// Returns StackType.O for reference types (this pointer passed as object reference),
+		/// and StackType.Ref for type parameters and value types (this pointer passed as managed reference).
+		/// </summary>
+		internal static StackType ExpectedTypeForThisPointer(IType type)
+		{
+			if (type.Kind == TypeKind.TypeParameter)
+				return StackType.Ref;
+			return type.IsReferenceType == true ? StackType.O : StackType.Ref;
+		}
+
+		internal override void CheckInvariant(ILPhase phase)
+		{
+			base.CheckInvariant(phase);
+			int firstArgument = (OpCode != OpCode.NewObj && !Method.IsStatic) ? 1 : 0;
+			Debug.Assert(Method.Parameters.Count + firstArgument == Arguments.Count);
+			if (firstArgument == 1) {
+				Debug.Assert(Arguments[0].ResultType == ExpectedTypeForThisPointer(ConstrainedTo ?? Method.DeclaringType),
+					$"Stack type mismatch in 'this' argument in call to {Method.Name}()");
+			}
+			for (int i = 0; i < Method.Parameters.Count; ++i) {
+				Debug.Assert(Arguments[firstArgument + i].ResultType == Method.Parameters[i].Type.GetStackType(),
+					$"Stack type mismatch in parameter {i} in call to {Method.Name}()");
+			}
+		}
+
 		public override void WriteTo(ITextOutput output, ILAstWritingOptions options)
 		{
 			ILRange.WriteTo(output, options);

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -437,6 +437,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				return (null, SpecialType.UnknownType);
 			IList<ILInstruction> arguments = null;
 			ILInstruction target = null;
+			IType targetType = null;
 			if (MatchGetMethodFromHandle(invocation.Arguments[0], out var member)) {
 				// static method
 				if (invocation.Arguments.Count != 2 || !MatchArgumentList(invocation.Arguments[1], out arguments)) {
@@ -447,13 +448,9 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					arguments = new List<ILInstruction>(invocation.Arguments.Skip(2));
 				}
 				if (!invocation.Arguments[0].MatchLdNull()) {
-					IType targetType;
 					(target, targetType) = ConvertInstruction(invocation.Arguments[0]);
 					if (target == null)
 						return (null, SpecialType.UnknownType);
-					if (targetType.IsReferenceType == false) {
-						target = new AddressOf(target);
-					}
 				}
 			}
 			if (arguments == null)
@@ -473,17 +470,37 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				}, delegateType);
 			}
 			CallInstruction call;
-			if (method.IsAbstract || method.IsVirtual || method.IsOverridable) {
+			if (method.IsAbstract || method.IsVirtual || method.IsOverride) {
 				call = new CallVirt(method);
 			} else {
 				call = new Call(method);
 			}
-			if (target != null)
-				call.Arguments.Add(target);
+			if (target != null) {
+				call.Arguments.Add(PrepareCallTarget(method.DeclaringType, target, targetType));
+			}
 			call.Arguments.AddRange(arguments);
 			return (call, method.ReturnType);
 		}
 
+		ILInstruction PrepareCallTarget(IType expectedType, ILInstruction target, IType targetType)
+		{
+			switch (CallInstruction.ExpectedTypeForThisPointer(expectedType)) {
+				case StackType.Ref:
+					if (target.ResultType == StackType.Ref)
+						return target;
+					else
+						return new AddressOf(target);
+				case StackType.O:
+					if (targetType.IsReferenceType == false) {
+						return new Box(target, targetType);
+					} else {
+						return target;
+					}
+				default:
+					return target;
+			}
+		}
+
 		(ILInstruction, IType) ConvertCast(CallInstruction invocation, bool isChecked)
 		{
 			if (invocation.Arguments.Count < 2)
@@ -651,7 +668,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return (null, SpecialType.UnknownType);
 				arguments[i] = arg;
 			}
-			var call = new Call(invokeMethod);
+			var call = new CallVirt(invokeMethod);
 			call.Arguments.Add(target);
 			call.Arguments.AddRange(arguments);
 			return (call, invokeMethod.ReturnType);
@@ -879,8 +896,9 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			if (invocation.Arguments.Count < 2)
 				return (null, SpecialType.UnknownType);
 			ILInstruction target = null;
+			IType targetType = null;
 			if (!invocation.Arguments[0].MatchLdNull()) {
-				target = ConvertInstruction(invocation.Arguments[0]).Item1;
+				(target, targetType) = ConvertInstruction(invocation.Arguments[0]);
 				if (target == null)
 					return (null, SpecialType.UnknownType);
 			}
@@ -896,15 +914,15 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						return (null, SpecialType.UnknownType);
 				}
 			}
-			if (target != null) {
-				arguments.Insert(0, target);
-			}
 			CallInstruction call;
-			if (member.IsAbstract || member.IsVirtual || member.IsOverridable) {
+			if (member.IsAbstract || member.IsVirtual || member.IsOverride) {
 				call = new CallVirt((IMethod)member);
 			} else {
 				call = new Call((IMethod)member);
 			}
+			if (target != null) {
+				call.Arguments.Add(PrepareCallTarget(member.DeclaringType, target, targetType));
+			}
 			call.Arguments.AddRange(arguments);
 			return (call, member.ReturnType);
 		}