Fix downcast check in slow array copy

When we have to resort to checking element by element, compare the type of each
actual element with the destination type. In particular, not the destinations
underlying type when it's an enum - we don't want to allow unrelated enums
using the same representation to copy over.

Fixes #64387

src/mono/System.Private.CoreLib/src/System/Array.Mono.cs

@@ -164,6 +164,7 @@ private static void Copy(Array sourceArray, int sourceIndex, Array destinationAr
 
             Type src_type = sourceArray.GetType().GetElementType()!;
             Type dst_type = destinationArray.GetType().GetElementType()!;
+            Type dst_elem_type = dst_type;
             bool dst_type_vt = dst_type.IsValueType && Nullable.GetUnderlyingType(dst_type) == null;
 
             bool src_is_enum = src_type.IsEnum;
@@ -196,12 +197,9 @@ private static void Copy(Array sourceArray, int sourceIndex, Array destinationAr
                 {
                     object srcval = sourceArray.GetValueImpl(source_pos + i);
 
-                    if (!src_type.IsValueType && dst_is_enum)
+                    if (dst_type_vt && (srcval == null || (src_type == typeof(object) && !dst_elem_type.IsAssignableFrom (srcval.GetType()))))
                         throw new InvalidCastException(SR.InvalidCast_DownCastArrayElement);
 
-                    if (dst_type_vt && (srcval == null || (src_type == typeof(object) && srcval.GetType() != dst_type)))
-                        throw new InvalidCastException();
-
                     try
                     {
                         destinationArray.SetValueRelaxedImpl(srcval, dest_pos + i);