For -ffast-math there was a missing using namespace __proposed left. The attached patch resolves the issue. From: Matthias Kretz The __bit_cast function was a hack to achieve what __builtin_bit_cast can do, therefore use __builtin_bit_cast if possible. However, __builtin_bit_cast cannot be used to cast from/to fixed_size_simd, since it isn't trivially copyable (in the language sense — in principle it is). Therefore add __proposed::simd_bit_cast to enable the use case required in the test framework. Signed-off-by: Matthias Kretz libstdc++-v3/ChangeLog: * include/experimental/bits/simd.h (__bit_cast): Implement via __builtin_bit_cast #if available. (__proposed::simd_bit_cast): Add overloads for simd and simd_mask, which use __builtin_bit_cast (or __bit_cast #if not available), which return an object of the requested type with the same bits as the argument. * include/experimental/bits/simd_math.h: Use simd_bit_cast instead of __bit_cast to allow casts to fixed_size_simd. (copysign): Remove branch that was only required if __bit_cast cannot be constexpr. * testsuite/experimental/simd/tests/bits/test_values.h: Switch from __bit_cast to __proposed::simd_bit_cast since the former will not cast fixed_size objects anymore. --- libstdc++-v3/include/experimental/bits/simd.h | 57 ++++++++++++++++++- .../include/experimental/bits/simd_math.h | 37 ++++++------ .../simd/tests/bits/test_values.h | 8 +-- 3 files changed, 76 insertions(+), 26 deletions(-) -- ────────────────────────────────────────────────────────────────────────── Dr. Matthias Kretz https://mattkretz.github.io GSI Helmholtz Centre for Heavy Ion Research https://gsi.de std::experimental::simd https://github.com/VcDevel/std-simd ──────────────────────────────────────────────────────────────────────────