Implement P0318R1 unwrap_ref_decay and unwrap_reference

Implement P0318R1 unwrap_ref_decay and unwrap_reference
	* include/std/type_traits (unwrap_reference, unwrap_reference_t)
	(unwrap_ref_decay, unwrap_ref_decay_t): New traits and aliases.
	* testsuite/20_util/unwrap_reference/1.cc: New test.
	* testsuite/20_util/unwrap_reference/2.cc: New test.

From-SVN: r266010

libstdc++-v3/ChangeLog

@@ -1,5 +1,11 @@
 2018-11-11  Jonathan Wakely  <jwakely@redhat.com>
 
+	Implement P0318R1 unwrap_ref_decay and unwrap_reference
+	* include/std/type_traits (unwrap_reference, unwrap_reference_t)
+	(unwrap_ref_decay, unwrap_ref_decay_t): New traits and aliases.
+	* testsuite/20_util/unwrap_reference/1.cc: New test.
+	* testsuite/20_util/unwrap_reference/2.cc: New test.
+
 	Implement P1007R3 std::assume_aligned
 	* include/std/memory (assume_aligned): Implement for C++17.
 	* testsuite/20_util/assume_aligned/1.cc: New test.

libstdc++-v3/include/std/type_traits

@@ -3015,6 +3015,20 @@ template <typename _From, typename _To>
   template<typename _Tp>
     using type_identity_t = typename type_identity<_Tp>::type;
 
+  /// Unwrap a reference_wrapper
+  template<typename _Tp>
+    struct unwrap_reference { using type = _Tp; };
+
+  template<typename _Tp>
+    struct unwrap_reference<reference_wrapper<_Tp>> { using type = _Tp&; };
+
+  /// Decay type and if it's a reference_wrapper, unwrap it
+  template<typename _Tp>
+    struct unwrap_ref_decay : unwrap_reference<decay_t<_Tp>> { };
+
+  template<typename _Tp>
+    using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
+
 #endif // C++2a
 
 _GLIBCXX_END_NAMESPACE_VERSION

libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc

@@ -0,0 +1,58 @@
+// Copyright (C) 2018 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 <type_traits>
+
+template<typename T, typename U> struct expect_same;
+template<typename T> struct expect_same<T, T> : std::true_type { };
+
+template<typename T, typename U = T>
+  constexpr bool check()
+  {
+    using std::unwrap_reference;
+    using T2 = typename unwrap_reference<T>::type;
+    static_assert(expect_same<T2, typename unwrap_reference<T2>::type>::value);
+    return expect_same<T2, U>::value;
+  }
+
+void
+test01()
+{
+  static_assert( check<int>() );
+  static_assert( check<const int>() );
+  static_assert( check<const int&>() );
+  static_assert( check<const int*>() );
+  static_assert( check<const int*&>() );
+
+  // reference_wrapper types should get unwrapped:
+  static_assert( check<std::reference_wrapper<int>, int&>() );
+  static_assert( check<std::reference_wrapper<const int>, const int&>() );
+  static_assert( check<std::reference_wrapper<long>, long&>() );
+
+  // But not cv-qualified reference_wrapper types:
+  static_assert( check<const std::reference_wrapper<int>>() );
+  static_assert( check<volatile std::reference_wrapper<int>>() );
+  static_assert( check<const volatile std::reference_wrapper<int>>() );
+
+  // Or references to reference_wrapper types:
+  static_assert( check<std::reference_wrapper<int>&>() );
+  static_assert( check<std::reference_wrapper<int>&&>() );
+  static_assert( check<const std::reference_wrapper<int>&>() );
+}

libstdc++-v3/testsuite/20_util/unwrap_reference/2.cc

@@ -0,0 +1,51 @@
+// Copyright (C) 2018 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 <type_traits>
+
+template<typename T, typename U> struct expect_same;
+template<typename T> struct expect_same<T, T> : std::true_type { };
+
+template<typename T, typename U = std::decay_t<T>>
+  constexpr bool check()
+  {
+    using T2 = typename std::unwrap_ref_decay<T>::type;
+    static_assert(expect_same<T2, std::unwrap_ref_decay_t<T>>::value);
+    return expect_same<T2, U>::value;
+  }
+
+void
+test01()
+{
+  static_assert( check<int>() );
+  static_assert( check<const int>() );
+  static_assert( check<const int&>() );
+  static_assert( check<const int*>() );
+  static_assert( check<const int*&>() );
+
+  // reference_wrapper types (including cv and references) get unwrapped:
+  static_assert( check<std::reference_wrapper<int>, int&>() );
+  static_assert( check<std::reference_wrapper<int>&, int&>() );
+  static_assert( check<const std::reference_wrapper<int>, int&>() );
+  static_assert( check<const std::reference_wrapper<int>&, int&>() );
+  static_assert( check<std::reference_wrapper<const int>, const int&>() );
+  static_assert( check<std::reference_wrapper<const int>&, const int&>() );
+  static_assert( check<std::reference_wrapper<long>, long&>() );
+}