Add workaround to std::variant for Clang bug 31852

	* include/std/variant (__get): Qualify calls to avoid ADL.
	(__select_index): Adjust whitespace.
	(variant): Add using-declaration to workaround Clang bug.

From-SVN: r258854

libstdc++-v3/ChangeLog

+2018-03-26  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/variant (__get): Qualify calls to avoid ADL.
+	(__select_index): Adjust whitespace.
+	(variant): Add using-declaration to workaround Clang bug.
+
 2018-03-22  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/85040

libstdc++-v3/include/std/variant

@@ -223,13 +223,17 @@ namespace __variant
 
   template<size_t _Np, typename _Union>
     constexpr decltype(auto) __get(in_place_index_t<_Np>, _Union&& __u)
-    { return __get(in_place_index<_Np-1>, std::forward<_Union>(__u)._M_rest); }
+    {
+      return __variant::__get(in_place_index<_Np-1>,
+			      std::forward<_Union>(__u)._M_rest);
+    }
 
   // Returns the typed storage for __v.
   template<size_t _Np, typename _Variant>
     constexpr decltype(auto) __get(_Variant&& __v)
     {
-      return __get(std::in_place_index<_Np>, std::forward<_Variant>(__v)._M_u);
+      return __variant::__get(std::in_place_index<_Np>,
+			      std::forward<_Variant>(__v)._M_u);
     }
 
   // Various functions as "vtable" entries, where those vtables are used by
@@ -358,10 +362,9 @@ namespace __variant
 
   template <typename... _Types>
   using __select_index =
-    typename __select_int::_Select_int_base<sizeof...(_Types)+1,
+    typename __select_int::_Select_int_base<sizeof...(_Types) + 1,
 					    unsigned char,
-					    unsigned short>
-    ::type::value_type;
+					    unsigned short>::type::value_type;
 
   template<typename... _Types>
     struct _Variant_storage<false, _Types...>
@@ -1304,6 +1307,12 @@ namespace __variant
 
 #undef _VARIANT_RELATION_FUNCTION_TEMPLATE
 
+#ifdef __clang__
+    public:
+      using _Base::_M_u; // See https://bugs.llvm.org/show_bug.cgi?id=31852
+    private:
+#endif
+
       template<size_t _Np, typename _Vp>
 	friend constexpr decltype(auto) __detail::__variant::__get(_Vp&& __v);