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

[Tim Song]

On Mon, May 22, 2017 at 9:21 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> 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>.
>

The thing that needs to be short-circuited out is the evaluation of
__accepted_type<_Tp&&>, which starts the tower of turtles.

The original patch does that (assuming core issue 1227's resolution),
but the __and_ version doesn't; __and_ only short circuits the
immediate parameter, not things used in forming it.