First half of support for N3657; heterogenous lookups for set/multiset

llvm-svn: 188241

1 files changed, 15 insertions, 0 deletions

diff --git a/libcxx/include/__functional_base b/libcxx/include/__functional_base
index 8ecc35b75da..72da75971b9 100644
--- a/libcxx/include/__functional_base
+++ b/libcxx/include/__functional_base
@@ -68,6 +68,7 @@ struct _LIBCPP_TYPE_VIS_ONLY less<void>
     template <class _T1, class _T2> _LIBCPP_INLINE_VISIBILITY
     auto operator()(_T1&& __t, _T2&& __u) const
         { return _VSTD::forward<_T1>(__t) < _VSTD::forward<_T2>(__u); }
+    typedef void is_transparent;
 };
 #endif
 
@@ -501,6 +502,20 @@ template <class _Tp> void cref(const _Tp&&);// = delete;
 
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
+#if _LIBCPP_STD_VER > 11
+template <class _Tp1, class _Tp2 = void>
+struct __is_transparent
+{
+private:
+    struct __two {char __lx; char __lxx;};
+    template <class _Up> static __two __test(...);
+    template <class _Up> static char __test(typename _Up::is_transparent* = 0);
+public:
+    static const bool value = sizeof(__test<_Tp1>(0)) == 1;
+};
+#endif
+
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_FUNCTIONAL_BASE