PR libstdc++/88802 define std::hash<nullptr_t> for C++17

	PR libstdc++/88802
	* include/bits/functional_hash.h (hash<nullptr_t>): Define
	specialization for C++17 (P0513R0, LWG 2817).
	* testsuite/20_util/hash/nullptr.cc: New test.

From-SVN: r267845

libstdc++-v3/ChangeLog

 2019-01-11  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/88802
+	* include/bits/functional_hash.h (hash<nullptr_t>): Define
+	specialization for C++17 (P0513R0, LWG 2817).
+	* testsuite/20_util/hash/nullptr.cc: New test.
+
 	PR libstdc++/88125
 	* config/abi/pre/gnu.ver (GLIBCXX_3.4.6): Remove unused duplicate
 	pattern for std::basic_stringbuf::str().

libstdc++-v3/include/bits/functional_hash.h

@@ -254,6 +254,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       operator()(long double __val) const noexcept;
     };
 
+#if __cplusplus >= 201703L
+  template<>
+    struct hash<nullptr_t> : public __hash_base<size_t, nullptr_t>
+    {
+      size_t
+      operator()(nullptr_t) const noexcept
+      { return 0; }
+    };
+#endif
+
   // @} group hashes
 
   // Hint about performance of hash functor. If not fast the hash-based

libstdc++-v3/testsuite/20_util/hash/nullptr.cc

+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
+// any later version.
+
+// This library 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.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+#include <functional>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  auto h1 = std::hash<std::nullptr_t>{}(nullptr);
+  static_assert(std::is_same_v<decltype(h1), std::size_t>);
+  auto h2 = std::hash<std::nullptr_t>{}(nullptr);
+  VERIFY( h1 == h2 );
+}
+
+int
+main()
+{
+  test01();
+}