re PR libstdc++/79513 (std::visit doesn't handle references)

	PR libstdc++/79513
	* include/std/variant (visit()): Forward variant types to the return
	type detection code.
	* testsuite/20_util/variant/compile.cc: Add test cases.

From-SVN: r245474

libstdc++-v3/ChangeLog

+2017-02-15  Tim Shen  <timshen@google.com>
+
+	PR libstdc++/79513
+	* include/std/variant (visit()): Forward variant types to the return
+	type detection code.
+	* testsuite/20_util/variant/compile.cc: Add test cases.
+
 2017-02-13  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR libstdc++/79348

libstdc++-v3/include/std/variant

@@ -1263,7 +1263,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__throw_bad_variant_access("Unexpected index");
 
       using _Result_type =
-	decltype(std::forward<_Visitor>(__visitor)(get<0>(__variants)...));
+	decltype(std::forward<_Visitor>(__visitor)(
+	    get<0>(std::forward<_Variants>(__variants))...));
 
       constexpr auto& __vtable = __detail::__variant::__gen_vtable<
 	_Result_type, _Visitor&&, _Variants&&...>::_S_vtable;

libstdc++-v3/testsuite/20_util/variant/compile.cc

@@ -291,6 +291,12 @@ void test_visit()
     };
     static_assert(visit(Visitor(), variant<int, nonliteral>(0)), "");
   }
+  // PR libstdc++/79513
+  {
+    std::variant<int> v [[gnu::unused]] (5);
+    std::visit([](int&){}, v);
+    std::visit([](int&&){}, std::move(v));
+  }
 }
 
 void test_constexpr()