Fix #1759: Substitute display-class type parameters with method type parameters in TransformDisplayClassUsage.

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

@@ -151,6 +151,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			void M3();
 		}
+
 		public class BaseClass : IM3
 		{
 			protected virtual void M1()
@@ -163,6 +164,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			{
 			}
 		}
+
 		public class SubClass : BaseClass
 		{
 			protected override void M2()
@@ -193,6 +195,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				Noop("M3", M3);
 #endif
 			}
+
 			public void Test2()
 			{
 				Noop("M3.new", new BaseClass().M3);
@@ -204,6 +207,28 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		public class GenericTest<TNonCaptured, TCaptured>
+		{
+			public Func<TCaptured> GetFunc(Func<TNonCaptured, TCaptured> f)
+			{
+				TCaptured captured = f(default(TNonCaptured));
+				return delegate {
+					Console.WriteLine(captured.GetType().FullName);
+					return captured;
+				};
+			}
+
+			public Func<TNonCaptured, TNonCapturedMP, TCaptured> GetFunc<TNonCapturedMP>(Func<TCaptured> f)
+			{
+				TCaptured captured = f();
+				return delegate(TNonCaptured a, TNonCapturedMP d) {
+					Console.WriteLine(a.GetHashCode());
+					Console.WriteLine(captured.GetType().FullName);
+					return captured;
+				};
+			}
+		}
+
 		public static Func<string, string, bool> test0 = (string a, string b) => string.IsNullOrEmpty(a) || string.IsNullOrEmpty(b);
 		public static Func<string, string, bool> test1 = (string a, string b) => string.IsNullOrEmpty(a) || !string.IsNullOrEmpty(b);
 		public static Func<string, string, bool> test2 = (string a, string b) => !string.IsNullOrEmpty(a) || string.IsNullOrEmpty(b);
@@ -342,5 +367,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			return (int b) => (int c) => (int d) => a + b + c + d;
 		}
+
+		public static Func<TCaptured> CapturedTypeParameter1<TNonCaptured, TCaptured>(TNonCaptured a, Func<TNonCaptured, TCaptured> f)
+		{
+			TCaptured captured = f(a);
+			return delegate {
+				Console.WriteLine(captured.GetType().FullName);
+				return captured;
+			};
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -321,7 +321,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					if (displayClass.IsMono && displayClass.CaptureScope == null && !IsOuterClosureReference(field)) {
 						displayClass.CaptureScope = BlockContainer.FindClosestContainer(inst);
 					}
-					v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, field.Type, field.Name);
+					v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, GetVariableTypeFromClosureField(field), field.Name);
 					v.HasInitialValue = true;
 					v.CaptureScope = displayClass.CaptureScope;
 					inst.ReplaceWith(new StLoc(v, inst.Value).WithILRange(inst));
@@ -370,7 +370,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				context.Step($"Introduce captured variable for {field.FullName}", inst);
 				// Introduce a fresh variable for the display class field.
 				Debug.Assert(displayClass.Definition == field.DeclaringTypeDefinition);
-				var v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, field.Type, field.Name);
+				var v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, GetVariableTypeFromClosureField(field), field.Name);
 				v.HasInitialValue = true;
 				v.CaptureScope = displayClass.CaptureScope;
 				inst.ReplaceWith(new LdLoca(v).WithILRange(inst));
@@ -381,5 +381,20 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				Debug.Fail("LdFlda pattern not supported!");
 			}
 		}
+
+		private IType GetVariableTypeFromClosureField(IField field)
+		{
+			if (!(field.Type is ITypeParameter typeParameter))
+				return field.Type;
+			var rootMethod = context.Function.Method;
+			if (typeParameter.Owner != field.DeclaringTypeDefinition)
+				return field.Type;
+			if (typeParameter.Index >= rootMethod.TypeParameters.Count) {
+				Debug.Assert(false, "Cannot map display-class type parameter to method type parameter");
+				return field.Type;
+			}
+
+			return rootMethod.TypeParameters[typeParameter.Index];
+		}
 	}
 }