[Bug libstdc++/125445] New: simd::vec conversion to underlying GNU vector type uses wrong type

[mkretz at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125445

            Bug ID: 125445
           Summary: simd::vec conversion to underlying GNU vector type
                    uses wrong type
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: mkretz at gcc dot gnu.org
          Reporter: mkretz at gcc dot gnu.org
  Target Milestone: ---

The _NativeVecType of simd::basic_vec incorrectly uses the canonical value type
to determine the implementation-defined vector type to be convertible to/from.

As a consequence, it is impossible to use a simd::vec<char, 32> where e.g.
__builtin_ia32_psadbw expects __vector(32) char.

Test case (https://compiler-explorer.com/z/ExedK6hs9):

#include <simd>
namespace simd = std::simd;

simd::vec<long long, 4> f(simd::vec<char, 32> x, simd::vec<char, 32> y)
{
    return __builtin_ia32_psadbw256(x, y);
}