https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107962
--- Comment #2 from Hubert Tong <hstong at ca dot ibm.com> --- https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2264 suggests that the GCC behaviour may be the desired one because copies of partially-initialized structures are allowed in C but cause undefined behaviour in C++.
