[Bug c++/94149] New: __is_constructible doesn't know about C++20 parenthesized init for arrays

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94149

            Bug ID: 94149
           Summary: __is_constructible doesn't know about C++20
                    parenthesized init for arrays
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

In C++20 this is well-formed:

  using T = int[2];
  T t(1, 2);

which means that std::is_constructible_v<int[2], int, int> should be true.

The FE intrinsic gives the wrong answer, and the std::is_nothrow_constructible
library trait isn't going to work even if the intrinsic starts to give the
right answer.


i.e. this should compile with -std=gnu++2a

#include <type_traits>

int main()
{
  using T = int[2];
  T t(1, 2);

  static_assert(__is_constructible(T, int, int));
  static_assert(std::is_constructible_v<T, int, int>);
  static_assert(std::is_nothrow_constructible_v<T, int, int>);

  return t[0];
}


a.cc: In function 'int main()':
a.cc:8:17: error: static assertion failed
    8 |   static_assert(__is_constructible(T, int, int));
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:9:22: error: static assertion failed
    9 |   static_assert(std::is_constructible_v<T, int, int>);
      |                 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:10:22: error: static assertion failed
   10 |   static_assert(std::is_nothrow_constructible_v<T, int, int>);
      |                 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~