Implement most of P1612R1: Relocate endian. Moves the std::endian functionality from 'type-traits' to 'bit'. No other change. The reason that this is 'partial' is that P1621 also recommends a feature-test macro, but I don't have the value for that one yet. In a month or so, I'll add that

llvm-svn: 366776

Author: mclow

libcxx/include/bit

@@ -42,6 +42,13 @@ namespace std {
   template<class T>
     constexpr int popcount(T x) noexcept;     // C++20
 
+  // 20.15.9, endian 
+  enum class endian {
+    little = see below,        // C++20
+    big = see below,           // C++20
+    native = see below         // C++20
+};
+
 } // namespace std
 
 */
@@ -456,6 +463,20 @@ log2p1(_Tp __t) noexcept
     return __t == 0 ? 0 : __bit_log2(__t) + 1;
 }
 
+
+enum class endian
+{
+    little = 0xDEAD,
+    big    = 0xFACE,
+#if defined(_LIBCPP_LITTLE_ENDIAN)
+    native = little
+#elif defined(_LIBCPP_BIG_ENDIAN)
+    native = big
+#else
+    native = 0xCAFE
+#endif
+};
+
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/type_traits

@@ -3985,21 +3985,6 @@ struct __can_extract_map_key<_ValTy, _Key, _Key, _RawValTy>
 
 #endif
 
-#if _LIBCPP_STD_VER > 17
-enum class endian
-{
-    little = 0xDEAD,
-    big    = 0xFACE,
-#if defined(_LIBCPP_LITTLE_ENDIAN)
-    native = little
-#elif defined(_LIBCPP_BIG_ENDIAN)
-    native = big
-#else
-    native = 0xCAFE
-#endif
-};
-#endif
-
 #ifndef _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED
 #if _LIBCPP_STD_VER > 17
 _LIBCPP_INLINE_VISIBILITY

libcxx/test/std/utilities/meta/meta.type.synop/endian.pass.cpp renamed to libcxx/test/std/numerics/bit/bit.endian/endian.pass.cpp

@@ -9,8 +9,9 @@
 // UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
 
 // enum class endian;
+// <bit>
 
-#include <type_traits>
+#include <bit>
 #include <cstring>
 #include <cassert>
 #include <cstdint>