[PATCH] Fix return type detection in <variant> visit()

[Jonathan Wakely]

On 14/02/17 13:59 -0800, Tim Shen via libstdc++ wrote:
>This is an obvious missing std::forward. :)

I was about to look into it, I assumed it would be something simple!

>diff --git a/libstdc++-v3/testsuite/20_util/variant/compile.cc b/libstdc++-v3/testsuite/20_util/variant/compile.cc
>index 65f4326c397..d40a4ccb784 100644
>--- a/libstdc++-v3/testsuite/20_util/variant/compile.cc
>+++ b/libstdc++-v3/testsuite/20_util/variant/compile.cc
>@@ -291,6 +291,13 @@ void test_visit()
>     };
>     static_assert(visit(Visitor(), variant<int, nonliteral>(0)), "");
>   }
>+  // PR libstdc++/79513
>+  {
>+    std::variant<int> v(5);
>+    std::visit([](int&){}, v);
>+    std::visit([](int&&){}, std::move(v));
>+    (void)v;

Is this to suppress an unused variable warning?

If it is, please use an attribute instead, as it's more reliable:

    std::variant<int> v __attribute__((unused)) (5);

OK for trunk if testing passes, thanks.