[release/6.0] [mono] Fix downcast check in Array.CopySlow (dotnet#64479)

* Add regression test for object[] -> Int32Enum[] array copy

where each element in the source array is the appropriate type

* 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 dotnet#64387

Co-authored-by: Aleksey Kliger <alklig@microsoft.com>

src/libraries/System.Runtime/tests/System/ArrayTests.cs

@@ -1275,6 +1275,9 @@ public static IEnumerable<object[]> Copy_SZArray_UnreliableConversion_CanPerform
 
             // Interface[] -> Class[]
             yield return new object[] { new NonGenericInterface1[10], 0, new NonGenericClass1[10], 0, 10, new NonGenericClass1[10] };
+
+            // object[] -> Int32Enum[] when values are all Int32Enum
+            yield return new object[] { new object[] { Int32Enum.Case3 }, 0, new Int32Enum[1], 0, 1, new Int32Enum[] { Int32Enum.Case3 } };
         }
 
         public static IEnumerable<object[]> Copy_Array_UnreliableConversion_CanPerform_TestData()

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);