Check for PreserveBaseOverridesAttribute

Covariant returns were introduced with .NET 5. For all earlier runtimes,
the last commit may have relaxed method signature comparison too much.

We can resolve this by checking for a specific custom attribute that
.NET compilers are expected to put on override methods using covariant
returns.

Reference:
https://github.com/dotnet/runtime/blob/main/docs/design/features/covariant-return-methods.md

Author: stakx

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

@@ -22,6 +22,8 @@ internal class MethodSignatureComparer : IEqualityComparer<MethodInfo>
 	{
 		public static readonly MethodSignatureComparer Instance = new MethodSignatureComparer();
 
+		private static readonly Type preserveBaseOverridesAttribute = Type.GetType("System.Runtime.CompilerServices.PreserveBaseOverridesAttribute", throwOnError: false);
+
 		public bool EqualGenericParameters(MethodInfo x, MethodInfo y)
 		{
 			if (x.IsGenericMethod != y.IsGenericMethod)
@@ -77,7 +79,7 @@ public bool EqualParameters(MethodInfo x, MethodInfo y)
 			return true;
 		}
 
-		public bool EqualSignatureTypes(Type x, Type y, bool precise = true)
+		public bool EqualSignatureTypes(Type x, Type y, MethodInfo xm = null)
 		{
 			if (x.IsGenericParameter != y.IsGenericParameter)
 			{
@@ -122,9 +124,12 @@ public bool EqualSignatureTypes(Type x, Type y, bool precise = true)
 			{
 				if (!x.Equals(y))
 				{
-					if (!precise && y.IsAssignableFrom(x))  // .NET 5+ covariant returns
+					// This enables covariant method returns for .NET 5 and newer.
+					// No need to check for runtime support, since such methods are marked with a custom attribute;
+					// see https://github.com/dotnet/runtime/blob/main/docs/design/features/covariant-return-methods.md.
+					if (xm != null && preserveBaseOverridesAttribute != null && xm.IsDefined(preserveBaseOverridesAttribute, inherit: false))
 					{
-						return true;
+						return y.IsAssignableFrom(x);
 					}
 
 					return false;
@@ -147,7 +152,7 @@ public bool Equals(MethodInfo x, MethodInfo y)
 
 			return EqualNames(x, y) &&
 				   EqualGenericParameters(x, y) &&
-				   EqualSignatureTypes(x.ReturnType, y.ReturnType, precise: false) &&
+				   EqualSignatureTypes(x.ReturnType, y.ReturnType, x) &&
 				   EqualParameters(x, y);
 		}