popcount uintN_t for N!=32 problem

[prlw1]

P2933R4 brings in overloads for <bit> functions.

Trying the following example (with g++ 16.0.0 20250827 -std=c++26 on NetBSD)

#include <bit>
#include <cstdint>
#include <print>
#include <simd>

int main() {
        using V = typename std::datapar::simd<std::uint8_t, 16>;
        V vec = std::datapar::iota<V>;
        for (std::size_t i{}; i != V::size; ++i) {
                std::println("{:4b}:{}", vec[i], std::popcount(vec[i]));
        }
/*
        auto pc = std::datapar::popcount(vec);
        for (std::size_t i{}; i != V::size; ++i) {
                std::println("{:4b}:{}", vec[i], pc[i]);
        }
*/
}

the above compiles and runs as is.

If the commented-out code is reinstated, compilation fails with

In file included from /usr/src/local/simd/simd:20,
                 from popcount.cc:4:
/usr/src/local/simd/bits/simd_bit.h: In instantiation of 'constexpr std::datapar::rebind_t<typename std::make_signed<typename _Vp::value_type>::type, _Vp> std::datapar::popcount(const _Vp&) [with _Vp = basic_simd<unsigned char, std::_VecAbi<16> >; rebind_t<typename std::make_signed<typename _Vp::value_type>::type, _Vp> = basic_simd<signed char, std::_VecAbi<16> >; typename std::make_signed<typename _Vp::value_type>::type = signed char; typename _Vp::value_type = unsigned char]':
required from here
popcount.cc:13:34:   
   13 |         auto pc = std::datapar::popcount(vec);
      |                   ~~~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/src/local/simd/bits/simd_bit.h:106:14: error: use of deleted function 'constexpr std::datapar::basic_simd<_Tp, _Abi>::basic_simd(_Fp&&) [with _Fp = std::datapar::popcount<basic_simd<unsigned char, std::_VecAbi<16> > >(const basic_simd<unsigned char, std::_VecAbi<16> >&)::<lambda(int)>; _Tp = signed char; _Abi = std::_VecAbi<16>]': Invalid return type of the generator function: Requires value-preserving conversion or implicitly convertible user-defined type.
  106 |       return _Rp([&](int __i) { return std::popcount(__v[__i]); });
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/src/local/simd/simd:12:
/usr/src/local/simd/bits/simd.h:166:9: note: declared here
  166 |         basic_simd(_Fp&& )
      |         ^~~~~~~~~~
/usr/src/local/simd/bits/simd_bit.h:106:14: note: use '-fdiagnostics-all-candidates' to display considered candidates
  106 |       return _Rp([&](int __i) { return std::popcount(__v[__i]); });
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
*** Error code 1

If uint8_t is replaced with uint32_t, then all is well again.

Apart from the principle of least surprise, I think

The type V::value_type is an unsigned integer type

should work for any uintN_t?

[prlw1]

This is still applicable to the rewrite branch bccd9eb

(Exciting to see your libstdc++ patch set yesterday!)

[mattkretz]

Thanks! I had not added unit tests for several [simd.bit] functions. The same issue exists for bit_width and the count[lr]_one/zero functions.