#1759: Follow-up: Handle nested generic arguments as well.

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

@@ -376,5 +376,14 @@ public static Func<TCaptured> CapturedTypeParameter1<TNonCaptured, TCaptured>(TN
 				return captured;
 			};
 		}
+
+		public static Func<TCaptured> CapturedTypeParameter2<TNonCaptured, TCaptured>(TNonCaptured a, Func<TNonCaptured, List<TCaptured>> f)
+		{
+			List<TCaptured> captured = f(a);
+			return delegate {
+				Console.WriteLine(captured.GetType().FullName);
+				return captured.FirstOrDefault();
+			};
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -303,6 +303,10 @@ protected internal override void VisitStObj(StObj inst)
 			// Get display class info
 			if (!IsDisplayClassLoad(target, out var displayClassLoad) || !displayClasses.TryGetValue(displayClassLoad, out var displayClass))
 				return;
+			// We want the specialized version, so that display-class type parameters are
+			// substituted with the type parameters from the use-site.
+			var fieldType = field.Type;
+			// However, use the unspecialized member definition to make reference comparisons in dictionary possible.
 			field = (IField)field.MemberDefinition;
 			if (displayClass.Variables.TryGetValue(field, out DisplayClassVariable info)) {
 				// If the display class field was previously initialized, we use a simple assignment.
@@ -321,7 +325,7 @@ protected internal override void VisitStObj(StObj inst)
 					if (displayClass.IsMono && displayClass.CaptureScope == null && !IsOuterClosureReference(field)) {
 						displayClass.CaptureScope = BlockContainer.FindClosestContainer(inst);
 					}
-					v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, GetVariableTypeFromClosureField(field), field.Name);
+					v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, fieldType, field.Name);
 					v.HasInitialValue = true;
 					v.CaptureScope = displayClass.CaptureScope;
 					inst.ReplaceWith(new StLoc(v, inst.Value).WithILRange(inst));
@@ -365,12 +369,16 @@ protected internal override void VisitLdFlda(LdFlda inst)
 			// Get display class info
 			if (!IsDisplayClassLoad(inst.Target, out var displayClassLoad) || !displayClasses.TryGetValue(displayClassLoad, out var displayClass))
 				return;
+			// Use the unspecialized member definition to make reference comparisons in dictionary possible.
 			var field = (IField)inst.Field.MemberDefinition;
+			// However, we want the specialized version, so that display-class type parameters are
+			// substituted with the type parameters from the use-site.
+			var fieldType = inst.Field.Type;
 			if (!displayClass.Variables.TryGetValue(field, out DisplayClassVariable info)) {
 				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, GetVariableTypeFromClosureField(field), field.Name);
+				var v = displayClass.DeclaringFunction.RegisterVariable(VariableKind.Local, fieldType, field.Name);
 				v.HasInitialValue = true;
 				v.CaptureScope = displayClass.CaptureScope;
 				inst.ReplaceWith(new LdLoca(v).WithILRange(inst));
@@ -381,20 +389,5 @@ protected internal override void VisitLdFlda(LdFlda inst)
 				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];
-		}
 	}
 }