[Patch] SFINAE on is_same first in variant's _Tp&& constructor

[Jonathan Wakely]

On 19/05/17 22:40 -0700, Tim Shen via libstdc++ wrote:
>diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
>index 0e04a820d69..b9824a5182c 100644
>--- a/libstdc++-v3/include/std/variant
>+++ b/libstdc++-v3/include/std/variant
>@@ -936,9 +936,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       noexcept((is_nothrow_move_constructible_v<_Types> && ...)) = default;
> 
>       template<typename _Tp,
>+	       typename = enable_if_t<!is_same_v<decay_t<_Tp>, variant>>,
> 	       typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
>-			  && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>
>-			  && !is_same_v<decay_t<_Tp>, variant>>>
>+			  && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>

Does this definitely short-circuit? I seem to recall a similar case
where either Clang or GCC (I think it was Clang) was evaluating the
second default template argument even though the first had produce a
substition failure.

If we need to guarantee it short-circuits then we'd want:

      template<typename _Tp,
 	       typename = enable_if_t<__and_<
                                      __not_<is_same<decay_t<_Tp>, variant>>,
                                      __bool_constant<
                                        __exactly_once<__accepted_type<_Tp&&>>
                                        && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>>>

i.e. use __and_< is-this-type, everything-else> where
"everything-else" still uses && to avoid making the instantiations too
deep.

Also, this is another place where we could use an __is_samey<T, U>
trait that does is_same<remove_cv_t<remove_reference_t<T>, U>.