> The following use of `std::bit_cast` has undefined behavior at compile time: > > ```cpp > constexpr auto x = std::bit_cast<__int128>(0.0L); // GCC accepts (x = 0), Clang rejects > ``` > > That is because an 80-bit x87 `long double` has 6 bytes of padding, and it is undefined behavior to map those padding bits onto non-padding bits in the destination type via `std::bit_cast`. [[bit.cast]](https://eel.is/c++draft/bit.cast) does not disqualify this use of `std::bit_cast` from being a constant expression.