[runtime] Disallow casts between int[] and IList<int?> etc.

mono/metadata/class.c

@@ -8411,13 +8411,15 @@ mono_class_is_assignable_from (MonoClass *klass, MonoClass *oklass)
 
 			// If the target we're trying to cast to is a valuetype, we must account of weird valuetype equivalences such as IntEnum <> int or uint <> int
 			// We can't apply it for ref types as this would go wrong with arrays - IList<byte[]> would have byte tested
-			if (iface_klass->valuetype)
-				iface_klass = iface_klass->cast_class;
+			if (!mono_class_is_nullable (iface_klass)) {
+				if (iface_klass->valuetype)
+					iface_klass = iface_klass->cast_class;
 
-			//array covariant casts only operates on scalar to scalar
-			//This is so int[] can't be casted to IComparable<int>[]
-			if (!(obj_klass->valuetype && !iface_klass->valuetype) && mono_class_is_assignable_from (iface_klass, obj_klass))
-				return TRUE;
+				//array covariant casts only operates on scalar to scalar
+				//This is so int[] can't be casted to IComparable<int>[]
+				if (!(obj_klass->valuetype && !iface_klass->valuetype) && mono_class_is_assignable_from (iface_klass, obj_klass))
+					return TRUE;
+			}
 		}
 
 		if (mono_class_has_variant_generic_params (klass)) {

mono/mini/objects.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Text;
 using System.Reflection;
+using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using System.Runtime.CompilerServices;
 
@@ -787,6 +788,11 @@ class Duper: Super {
 		}
 		if (!ok)
 			return 12;
+
+		object arr = new int [10];
+		if (arr is IList<int?>)
+			return 13;
+
 		return 0;
 	}