[GedByrne]

Then why isn't the code being disallowed? Why is it being silently removed in the middle of the night through an optimization back door?

[pornel]

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.