Re: [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>.