Unsupported `constexpr std::bit_cast` for `std::complex`

[Svalorzen]

Hello, I am trying to bit_cast a structure containing std::complex, and clang is reporting that this is unsupported. I'm attaching here an MVE:

#include <bit>
#include <iostream>
#include <complex>
#include <array>

using C = std::complex<double>;

int main() {
    constexpr unsigned char data[sizeof(C)] = {0};

    constexpr C c = std::bit_cast<C>(data);
    std::cout << c << '\n';

    return 0;
}

This seems to fail on trunk as per godbolt (https://godbolt.org/z/q8fKE7r18).

There exists a similar issue, #54018, with PR #74775, which involves bit-fields. I am not sure whether this is related at low level, but I think enabling std::bit_cast on a "native" standard library type is quite important. I am opening this issue as I have not been able to find any status reports on this problem.

[c8ef]

If you use libc++ instead of libstdc++, the program above works perfectly.

https://godbolt.org/z/59GY8azhM

[Svalorzen]

That's encouraging. Any chance it would work with libstdc++ as well?

[c8ef]

Actually latest gcc also support this: https://godbolt.org/z/s8dd5dxfd.

Perhaps it would be better for us to use the standard library provided by the corresponding compilers.

[Svalorzen]

I am on Linux, and I confirm that I can compile this just fine with gcc. The only reason I discovered this problem is because I use YCM with vim, which uses LLVM in order to provide diagnostics in editor. So I started getting this error reported, then I tried to compile using clang (which on Linux I think uses stdlibc++ by default) and I confirmed the problem.

Installing libc++ on Linux is not necessarily easy so that's why I was asking whether it would be possible for clang to make this work with libstdc++ as well.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eugenio Bargiacchi (Svalorzen)

Hello, I am trying to bit_cast a structure containing `std::complex`, and clang is reporting that this is unsupported. I'm attaching here an MVE: ``` #include <bit> #include <iostream> #include <complex> #include <array>

using C = std::complex<double>;

int main() {
constexpr unsigned char data[sizeof(C)] = {0};

constexpr C c = std::bit_cast<C>(data);
std::cout << c << '\n';

return 0;

}

This seems to fail on trunk as per godbolt (https://godbolt.org/z/q8fKE7r18).

There exists a similar issue, #<!-- -->54018, with PR #<!-- -->74775, which involves bit-fields. I am not sure whether this is related at low level, but I think enabling `std::bit_cast` on a "native" standard library type is quite important. I am opening this issue as I have not been able to find any status reports on this problem.
</details>

[tbaederr]

This is simply a problem with clang's implementation of __builtin_bit_cast.

Much simpler reproducer:

constexpr _Complex short A = {1, 2};
constexpr int B = __builtin_bit_cast(int, A);

https://godbolt.org/z/YErW4s5ed

The PR you linked probably doesn't help but fixing this bug here should be much simpler than implementing bit casts for bit fields.