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Undefined behavior is not illegal. The compiler can do anything with undefined behavior, including exactly what the author expected.

It is illegal, for any reasonable definition of illegal. See my comment from earlier in the year: https://news.ycombinator.com/item?id=10840497

"Illegal" seems stronger to me than "not strictly conforming", I guess (and so does "well-formed", for that matter). But I think we basically agree.