Implement std::to_address for C++2a

diff --git a/libstdc++-v3/include/bits/ptr_traits.h b/libstdc++-v3/include/bits/ptr_traits.h
index 74d4c18126c..3334c0c9069 100644
--- a/libstdc++-v3/include/bits/ptr_traits.h
+++ b/libstdc++-v3/include/bits/ptr_traits.h
@@ -151,10 +151,49 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __to_address(_Tp* __ptr) noexcept
     { return __ptr; }
 
+#if __cplusplus <= 201703L
   template<typename _Ptr>
     constexpr typename std::pointer_traits<_Ptr>::element_type*
     __to_address(const _Ptr& __ptr)
     { return std::__to_address(__ptr.operator->()); }
+#else
+  template<typename _Ptr>
+    constexpr auto
+    __to_address(const _Ptr& __ptr) noexcept
+    -> decltype(std::pointer_traits<_Ptr>::to_address(__ptr))
+    { return std::pointer_traits<_Ptr>::to_address(__ptr); }
+
+  template<typename _Ptr, typename... _None>
+    constexpr auto
+    __to_address(const _Ptr& __ptr, _None...) noexcept
+    { return std::__to_address(__ptr.operator->()); }
+
+  /**
+   * @brief Obtain address referenced by a pointer to an object
+   * @param __ptr A pointer to an object
+   * @return @c __ptr
+   * @ingroup pointer_abstractions
+  */
+  template<typename _Tp>
+    constexpr _Tp*
+    to_address(_Tp* __ptr) noexcept
+    {
+      static_assert(!std::is_function<_Tp>::value, "not a function pointer");
+      return __ptr;
+    }
+
+  /**
+   * @brief Obtain address referenced by a pointer to an object
+   * @param __ptr A pointer to an object
+   * @return @c pointer_traits<_Ptr>::to_address(__ptr) if that expression is
+             well-formed, otherwise @c to_address(__ptr.operator->())
+   * @ingroup pointer_abstractions
+  */
+  template<typename _Ptr>
+    constexpr auto
+    to_address(const _Ptr& __ptr) noexcept
+    { return std::__to_address(__ptr); }
+#endif // C++2a
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
diff --git a/libstdc++-v3/testsuite/20_util/to_address/1.cc b/libstdc++-v3/testsuite/20_util/to_address/1.cc
new file mode 100644
index 00000000000..507d20e18a8
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/to_address/1.cc
@@ -0,0 +1,146 @@ 
+// Copyright (C) 2011-2017 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 run { target c++2a } }
+
+#include <memory>
+#include <testsuite_hooks.h>
+
+class P1
+{
+public:
+  using element_type = int;
+
+  explicit P1(int* p)
+  : p_(p) { }
+
+  int* operator->() const noexcept
+  { return p_; }
+
+private:
+  int* p_;
+};
+
+class P2
+{
+public:
+  using element_type = int;
+
+  explicit P2(int* p)
+  : p_(p) { }
+
+  P1 operator->() const noexcept
+  { return p_; }
+
+private:
+  P1 p_;
+};
+
+class P3
+{
+public:
+  explicit P3(int* p)
+  : p_(p) { }
+
+  int* get() const noexcept
+  { return p_; }
+
+private:
+  int* p_;
+};
+
+namespace std
+{
+  template<>
+    struct pointer_traits<::P3>
+    {
+      static int* to_address(const ::P3& p) noexcept
+      { return p.get(); }
+    };
+}
+
+class P4
+{
+public:
+  explicit P4(int* p)
+  : p_(p) { }
+
+  int* operator->() const noexcept
+  { return nullptr; }
+
+  int* get() const noexcept
+  { return p_; }
+
+private:
+  int* p_;
+};
+
+namespace std
+{
+  template<>
+    struct pointer_traits<::P4>
+    {
+      static int* to_address(const ::P4& p) noexcept
+      { return p.get(); }
+    };
+}
+
+void test01()
+{
+  int i = 0;
+  int* p = &i;
+  VERIFY( std::to_address(p) == &i );
+}
+
+void test02()
+{
+  int i = 0;
+  P1 p(&i);
+  VERIFY( std::to_address(p) == &i );
+}
+
+void test03()
+{
+  int i = 0;
+  P2 p(&i);
+  VERIFY( std::to_address(p) == &i );
+}
+
+void test04()
+{
+  int i = 0;
+  P3 p(&i);
+  VERIFY( std::to_address(p) == &i );
+}
+
+void test05()
+{
+  int i = 0;
+  P4 p(&i);
+  VERIFY( std::to_address(p) == &i );
+}
+
+int main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+  test05();
+  return 0;
+}