Given that __underlying_type is now available in clang, implement

std::underlying_type.

llvm-svn: 135410

libcxx/include/__config

@@ -188,6 +188,10 @@ namespace std {
 #  define _NOEXCEPT_(x)
 #endif
 
+#if __has_feature(underlying_type)
+#  define _LIBCXX_UNDERLYING_TYPE(T) __underlying_type(T)
+#endif
+
 // end defined(__clang__)
 
 #elif defined(__GNUC__)

libcxx/include/type_traits

@@ -3092,6 +3092,26 @@ struct __is_nothrow_swappable
 
 #endif  // __has_feature(cxx_noexcept)
 
+#ifdef _LIBCXX_UNDERLYING_TYPE
+
+template <class _Tp>
+struct underlying_type
+{
+    typedef _LIBCXX_UNDERLYING_TYPE(_Tp) type;
+};
+
+#else  // _LIBCXX_UNDERLYING_TYPE
+
+template <class _Tp, bool _Support = false>
+struct underlying_type
+{
+    static_assert(_Support, "The underyling_type trait requires compiler "
+                            "support. Either no such support exists or "
+                            "libc++ does not know how to use it.");
+};
+
+#endif // _LIBCXX_UNDERLYING_TYPE
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_TYPE_TRAITS

libcxx/test/utilities/meta/meta.trans/meta.trans.other/underlying_type.pass.cpp

@@ -12,8 +12,22 @@
 // underlying_type
 
 #include <type_traits>
+#include <climits>
 
 int main()
 {
-#error underlying_type is not implemented
+    enum E { V = INT_MIN };
+    enum F { W = UINT_MAX };
+
+    static_assert((std::is_same<std::underlying_type<E>::type, int>::value),
+                  "E has the wrong underlying type");
+    static_assert((std::is_same<std::underlying_type<F>::type, unsigned>::value),
+                  "F has the wrong underlying type");
+
+#if __has_feature(cxx_strong_enums)
+    enum G : char { };
+
+    static_assert((std::is_same<std::underlying_type<G>::type, char>::value),
+                  "G has the wrong underlying type");
+#endif // __has_feature(cxx_strong_enums)
 }

libcxx/www/type_traits_design.html

@@ -260,7 +260,7 @@ changed.  Please see:
 
 <tr>
 <td><tt>underlying_type&lt;T&gt;</tt></td>
-<td bgcolor="#FF5965"><tt>__underlying_type(T)</tt></td>
+<td bgcolor="#80FF80"><tt>__underlying_type(T)</tt></td>
 </tr>
 
 <tr>