[PATCH] Define std::endian for C++2a (P0463R1)

[Jonathan Wakely]

This is a tiny feature, small but perfectly formed. And easy to
implement.

        * include/std/type_traits (endian): Define new enumeration type.
        * testsuite/20_util/endian/1.cc: New test.

Tested powerpc64le-linux, committed to trunk.


commit ad69c5875cdefc5c699b50975ee6016a424d450b
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Nov 3 15:04:51 2017 +0000

    Define std::endian for C++2a (P0463R1)
    
            * include/std/type_traits (endian): Define new enumeration type.
            * testsuite/20_util/endian/1.cc: New test.

diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index 036f7667bd8..7eca08c1a50 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2664,7 +2664,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     void operator=(__nonesuch const&) = delete;
   };
 
-#if __cplusplus > 201402L
+#if __cplusplus >= 201703L
 # define __cpp_lib_is_invocable 201703
 
   /// std::invoke_result
@@ -2739,7 +2739,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       = is_nothrow_invocable_r<_Fn, _Args...>::value;
 #endif // C++17
 
-#if __cplusplus > 201402L
+#if __cplusplus >= 201703L
 # define __cpp_lib_type_trait_variable_templates 201510L
 template <typename _Tp>
   inline constexpr bool is_void_v = is_void<_Tp>::value;
@@ -2943,6 +2943,16 @@ template <typename _From, typename _To>
 
 #endif // C++17
 
+#if __cplusplus > 201703L
+  /// Byte order
+  enum class endian
+  {
+    little = __ORDER_LITTLE_ENDIAN__,
+    big    = __ORDER_BIG_ENDIAN__,
+    native = __BYTE_ORDER__
+  };
+#endif // C++2a
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 
diff --git a/libstdc++-v3/testsuite/20_util/endian/1.cc 
b/libstdc++-v3/testsuite/20_util/endian/1.cc
new file mode 100644
index 00000000000..2720c5edcdb
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/endian/1.cc
@@ -0,0 +1,36 @@
+// Copyright (C) 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 compile { target c++2a } }
+
+#include <type_traits>
+
+static_assert( std::is_enum_v<std::endian> );
+static_assert( std::endian::little != std::endian::big );
+static_assert( std::endian::native == std::endian::big
+               || std::endian::native == std::endian::little );
+
+namespace gnu {
+  int little, big, native;
+}
+
+using namespace std;
+using namespace gnu;
+
+// std::endian is a scoped-enum so these should refer to gnu::native etc.
+int test = little + big + native;