Copy Custom Modifiers instead of handling init property implicitly

Author: hazzik

src/NHibernate.Test/StaticProxyTest/IsExternalInit.cs

@@ -1,5 +1,7 @@
-using System.ComponentModel;
+#if !NET5_0_OR_GREATER
+using System.ComponentModel;
 
+// ReSharper disable once CheckNamespace
 namespace System.Runtime.CompilerServices
 {
 	/// <summary>
@@ -11,3 +13,4 @@ internal static class IsExternalInit
 	{
 	}
 }
+#endif

src/NHibernate/Proxy/DefaultDynamicProxyMethodCheckerExtensions.cs

@@ -17,25 +17,17 @@ public static bool IsProxiable(this MethodInfo method)
 				((method.IsPublic || method.IsFamily) && (method.IsVirtual || method.IsAbstract)) // public or protected (virtual)
 				||
 				(method.IsFamilyOrAssembly && (method.IsVirtual || method.IsAbstract)) // internal protected virtual
-				)
-				&& !method.IsPropertyInit();
-		}
-
-		public static bool IsPropertyInit(this MethodInfo method)
-		{
-			return method.IsSpecialName &&
-				method.IsHideBySig &&
-				method.ReturnParameter.GetRequiredCustomModifiers().Any(n => n.FullName == "System.Runtime.CompilerServices.IsExternalInit");
+				);
 		}
 
 		public static bool ShouldBeProxiable(this MethodInfo method)
 		{
 			// to use only for real methods (no getter/setter)
 			return (method.DeclaringType != typeof (MarshalByRefObject)) &&
-				   (method.DeclaringType != typeof (object) || !"finalize".Equals(method.Name, StringComparison.OrdinalIgnoreCase)) &&
-				   (!(method.DeclaringType == typeof (object) && "GetType".Equals(method.Name))) &&
-				   (!(method.DeclaringType == typeof (object) && "obj_address".Equals(method.Name))) && // Mono-specific method
-				   !IsDisposeMethod(method) &&
+			       (method.DeclaringType != typeof (object) || !"finalize".Equals(method.Name, StringComparison.OrdinalIgnoreCase)) &&
+			       (!(method.DeclaringType == typeof (object) && "GetType".Equals(method.Name))) &&
+			       (!(method.DeclaringType == typeof (object) && "obj_address".Equals(method.Name))) && // Mono-specific method
+			       !IsDisposeMethod(method) &&
 						 (method.IsPublic || method.IsAssembly || method.IsFamilyOrAssembly);
 		}
 

src/NHibernate/Proxy/ProxyBuilderHelper.cs

@@ -171,13 +171,18 @@ internal static MethodBuilder GenerateMethodSignature(string name, MethodInfo me
 			var implementationName = explicitImplementation
 				? $"{method.DeclaringType.FullName}.{name}"
 				: name;
+
 			var methodBuilder =
 				typeBuilder.DefineMethod(
 					implementationName,
 					methodAttributes,
 					CallingConventions.HasThis,
 					method.ReturnType,
-					parameters.ToArray(param => param.ParameterType));
+					method.ReturnParameter?.GetRequiredCustomModifiers(),
+					method.ReturnParameter?.GetOptionalCustomModifiers(),
+					parameters.ToArray(p => p.ParameterType),
+					parameters.ToArray(p => p.GetRequiredCustomModifiers()),
+					parameters.ToArray(p => p.GetOptionalCustomModifiers()));
 
 			var typeArgs = method.GetGenericArguments();
 			if (typeArgs.Length > 0)