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’