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Re: [Patch] SFINAE on is_same first in variant's _Tp&& constructor
- From: Tim Song <t dot canens dot cpp at gmail dot com>
- To: Jonathan Wakely <jwakely at redhat dot com>
- Cc: Tim Shen <timshen at google dot com>, "libstdc++" <libstdc++ at gcc dot gnu dot org>, gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Mon, 22 May 2017 16:14:01 -0400
- Subject: Re: [Patch] SFINAE on is_same first in variant's _Tp&& constructor
- Authentication-results: sourceware.org; auth=none
- References: <CAG4ZjNkZui6RkKB-cydS+q1=xUcuNS42Uha6w-N7fK7GbEOmSw@mail.gmail.com> <20170522132137.GH4527@redhat.com>
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.