I don't know about the standard, but if cppreference.com is good enough: At https://en.cppreference.com/w/cpp/language/union https://en.cppreference.com/w/cpp/l

I don't know about the standard, but if cppreference.com is good enough: At https://en.cppreference.com/w/cpp/language/union https://en.cppreference.com/w/cpp/language/union it says "If two union members are standard-layout types, it's well-defined to examine their common subsequence on any compiler."