It is a term of art in compilers/language design though, isn't it? If you break an invariant the compiler is relying on for optimization then you can't say for

It is a term of art in compilers/language design though, isn't it? If you break an invariant the compiler is relying on for optimization then you can't say for sure what the effect after all optimisation passes or in future versions of the compiler will be. It's just "undefined"