[Bug c++/114536] New: wrong constant evaluation of std::bit_cast for bit fields

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114536

            Bug ID: 114536
           Summary: wrong constant evaluation of std::bit_cast for bit
                    fields
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

In this program


#include <bit>

struct A { unsigned char a: 7; };

struct B { unsigned char b; };

// fails in GCC
static_assert( std::bit_cast<B>(A{1}).b == 1 );

int main() {
    // correctly returns 1 in GCC
    return std::bit_cast<B>(A{1}).b;
}


static_assert fails with the message:

note: the comparison reduces to '(0 == 1)'

which contradicts runtime evaluation of the same expression. Online demo:
https://gcc.godbolt.org/z/fza4zas3E