That cannot work. std::bit_cast_clear_padding would clear the padding bits in the input that would have otherwise resulted in an indeterminate output, but the padding bits of the output are unspecified. Due to ABI constraints, unless std::bit_cast has custom ABI, that is set in stone.

Consider that when returning an empty struct from any function, including std::bit_cast, the ABI on some platforms is to not pass anything into any register (nor spill into memory), so padding bits don't exist. Even if the input to std::bit_cast was a zeroed byte array, that has no impact on the result. I suppose this could be avoided if we said that std::bit_cast was not a function in the first place; the intrinsic __builtin_bit_cast is not constrained by ABI.

Ergo, if you want something that clears padding in-place, that requires a std::clear_padding function, unless you treat std::bit_cast magically and give it custom ABI.