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


On Mon, May 22, 2017 at 6:21 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.

Good observation. I changed to use __and_ and __not_:

-              typename = enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
-                         && is_constructible_v<__accepted_type<_Tp&&>, _Tp&&>
-                         && !is_same_v<decay_t<_Tp>, variant>>>
+              typename = enable_if_t<__and_<
+                             __not_<is_same<decay_t<_Tp>, variant>>,
+                             std::integral_constant<bool,
+                                 __exactly_once<__accepted_type<_Tp&&>>>,
+                             is_constructible<__accepted_type<_Tp&&>,
_Tp&&>>::value>>

(I didn't use && at all, just to verify the correctness)

but the compile still fails with the similar error messages. If __and_
and __not_ are expected to work, then the root cause is unlikely "not
short-circuit" only.

I suggest to cc a front-end person (Jason?) to take a look, as I
suggested in the bug, and the example: https://godbolt.org/g/AxUv16.

>
> 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>.
>

I never know that "samey" is a word. :)


-- 
Regards,
Tim Shen


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