--- old/src/hotspot/share/oops/array.hpp	2018-08-06 13:58:49.374951635 -0700
+++ new/src/hotspot/share/oops/array.hpp	2018-08-06 13:58:49.026938517 -0700
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -132,7 +132,7 @@
 
     return (int)words;
   }
-  int size() {
+  int size() const {
     return size(_length);
   }
 
@@ -153,6 +153,25 @@
   }
   void print() { print(tty); }
 #endif // PRODUCT
+
+  // Returns a "const" Array of S, where S is a supertype of T.
+  //
+  //     Array<InstanceKlass*>* ta = ... ;
+  //     Array<Klass*>* sa = ta->as_array_of<Klass*>();
+  //     Klass* k = sa->at(0);  /// OK to read
+  //     Klass* ak = k->array_klass(2);
+  //     sa->at_put(0, ak);     ///  NOT OK to write into it, or else
+  //                            /// ta would contain something unexpected.
+  //
+  // If you get a C compilation error, DO NOT simply cast the 'const' away!
+  template <typename S> const Array<S>* as_const_array_of() {
+#ifndef PRODUCT
+    T dummy = NULL;
+    S super_dummy = dummy; // must be down-castable
+    assert(sizeof(T) == sizeof(S), "must be");
+#endif 
+    return static_cast<Array<S>*>((void*)this);
+  }
 };