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diff --git a/libgfortran/intrinsics/extends_type_of.c b/libgfortran/intrinsics/extends_type_of.c
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+++ b/libgfortran/intrinsics/extends_type_of.c
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+/* Implementation of the EXTENDS_TYPE_OF intrinsic.
+   Copyright (C) 2004-2017 Free Software Foundation, Inc.
+   Contributed by Janus Weil <janus@gcc.gnu.org>.
+
+This file is part of the GNU Fortran runtime library (libgfortran).
+
+Libgfortran is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public
+License as published by the Free Software Foundation; either
+version 3 of the License, or (at your option) any later version.
+
+Libgfortran is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */ 
+
+
+#include "libgfortran.h"
+
+
+typedef struct vtype
+{
+  GFC_INTEGER_4 hash;
+  GFC_INTEGER_4 size;
+  struct vtype *extends;
+}
+vtype;
+
+
+extern GFC_LOGICAL_4 is_extension_of (struct vtype *, struct vtype *);
+export_proto(is_extension_of);
+
+
+/* This is a helper function for the F2003 intrinsic EXTENDS_TYPE_OF.
+   While EXTENDS_TYPE_OF accepts CLASS or TYPE arguments, this one here gets
+   passed the corresponding vtabs. Each call to EXTENDS_TYPE_OF is translated
+   to a call to is_extension_of.  */
+
+GFC_LOGICAL_4
+is_extension_of (struct vtype *v1, struct vtype *v2)
+{
+  /* Assume that only unlimited polymorphic entities will pass NULL v1 or v2
+     if they are unallocated or disassociated.  */
+
+  if (!v2)
+    return 1;
+  if (!v1)
+    return 0;
+
+  while (v1)
+    {
+      if (v1->hash == v2->hash) return 1;
+      v1 = v1->extends;
+    }
+  return 0;
+}