It is disallowed by the standard. It's not disallowed by compilers, because they're deficient. In this case I'd guess compiler front-end doesn't track which va

It is disallowed by the standard. It's not disallowed by compilers, because they're deficient. In this case I'd guess compiler front-end doesn't track which values in an array are initialized, and when optimizing back-end sees UB it's too late to emit an error.