https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586
--- Comment #21 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Created attachment 52413 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52413&action=edit gcc12-pr102586.patch Indeed, and when creating BIT_CAST_EXPR we actually verify those trivially_copyable_p types. This patch implements the rejection of pointers to non-trivially-copyable types with the exception of &var or &parm, because in those cases we know it is complete object type and so don't need to worry about it being just a subobject. I think the exception for that case is useful, so that users can still clear the padding bits on them, but the restriction means that they need to use the builtin directly or in a macro rather than through inline function. But the risks of clobbering real data otherwise is real. The other option would be just document that it shouldn't be called on non-trivially-copyable base classes and make it user's responsibility. For the <atomic> and std::bit_cast uses it doesn't really matter.
