Minimal bugfix: relax return type equality check

The previous commit's test fails due to DynamicProxy treating the base
and derived record classes' `<Clone>$` methods as two distinct methods
(because they differ in their exact return type), instead of as a single
overridden method. We can change this by adjusting `MethodSignature-
Comparer` such that it also accepts assignment-compatible return types.

Author: stakx

src/Castle.Core/DynamicProxy/Generators/MethodSignatureComparer.cs

@@ -77,7 +77,7 @@ public bool EqualParameters(MethodInfo x, MethodInfo y)
 			return true;
 		}
 
-		public bool EqualSignatureTypes(Type x, Type y)
+		public bool EqualSignatureTypes(Type x, Type y, bool precise = true)
 		{
 			if (x.IsGenericParameter != y.IsGenericParameter)
 			{
@@ -122,6 +122,11 @@ public bool EqualSignatureTypes(Type x, Type y)
 			{
 				if (!x.Equals(y))
 				{
+					if (!precise && y.IsAssignableFrom(x))  // .NET 5+ covariant returns
+					{
+						return true;
+					}
+
 					return false;
 				}
 			}
@@ -142,7 +147,7 @@ public bool Equals(MethodInfo x, MethodInfo y)
 
 			return EqualNames(x, y) &&
 				   EqualGenericParameters(x, y) &&
-				   EqualSignatureTypes(x.ReturnType, y.ReturnType) &&
+				   EqualSignatureTypes(x.ReturnType, y.ReturnType, precise: false) &&
 				   EqualParameters(x, y);
 		}