[Bug libstdc++/114803] simd conversion to [[gnu::vector_size(N)]] type hits invalid code in experimental/bits/simd_builtin.h

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114803

--- Comment #10 from Alexandre Oliva <aoliva at gcc dot gnu.org> ---
FWIW, I can trigger the problem on arm-eabi (and presumably also on
aarch64-elf, but I haven't tried that) with -D__clang__.

(our vxworks toolchains have to define that macro, for complicated reasons)

here's a reduced testcase:

arm-eabi-g++ ~/pr114803.cc -mfpu=neon -mfloat-abi=hard -O0 -std=gnu++17
-D__clang__ [-funsigned-char]

include <experimental/simd>

template <typename T>
void
test()
{
  using V = std::experimental::simd<T,
std::experimental::simd_abi::_VecBuiltin<16> >;
  if constexpr (std::is_signed_v<T>)
    static_cast<int8x16_t>(V{});
  else
    static_cast<uint8x16_t>(V{});
}

int main()
{
  test<char>();
}

pr114803.cc: In instantiation of ‘void test() [with T = char]’:
pr114803.cc:16:   required from here
pr114803.cc:11: error: invalid ‘static_cast’ from type
‘std::experimental::parallelism_v2::simd<char,
std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16> >’ to type
‘uint8x16_t’