Compilers are not able to prevent you from violating must/shall in the general case. So they're not held to that bar. Unless the standard says not to compile it, it's not a compiler bug.

Also, imagine a situation where the line of code actually lists three different variables, but all three of them are passed in by address. It quickly becomes impossible for the compiler to know you violated the spec by reusing the same variable. And even optimizations that make sense here could corrupt the value pretty badly and possibly lead to worse errors.