Constexpr in std::pointer_traits (P1006R1)

	* include/bits/ptr_traits.h [C++20] (pointer_traits<T*>::pointer_to):
	Add constexpr.
	* testsuite/20_util/pointer_traits/pointer_to_constexpr.cc: New test.

From-SVN: r269418

libstdc++-v3/ChangeLog

+2019-03-06  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/ptr_traits.h [C++20] (pointer_traits<T*>::pointer_to):
+	Add constexpr.
+	* testsuite/20_util/pointer_traits/pointer_to_constexpr.cc: New test.
+
 2019-03-05  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/c_compatibility/math.h [C++20] (lerp): Add using

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

@@ -137,7 +137,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  @param  __r  A reference to an object of type @c element_type
        *  @return @c addressof(__r)
       */
-      static pointer
+      static _GLIBCXX20_CONSTEXPR pointer
       pointer_to(__make_not_void<element_type>& __r) noexcept
       { return std::addressof(__r); }
     };

libstdc++-v3/testsuite/20_util/pointer_traits/pointer_to_constexpr.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++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <memory>
+
+int i = 0;
+static_assert( std::pointer_traits<int*>::pointer_to(i) == &i );
+
+struct X { } x;
+static_assert( std::pointer_traits<X*>::pointer_to(x) == &x );