Nope, "undefined" leaves the compiler perfectly free to just Do The Right Thing.

I didn't make it up in my head - I observed it in working code. Widely used working code. And the compiler is not at all forced to assume x is not null. The standard leaves it to the compiler writer to handle the case. Could the compiler perform sophisticated static analysis and reject the code under the standard? Yes. Could the compiler simply compile the code as written ? Yes. Could the compiler abuse the vagueness of the standard to produce an "optimization" that implements something the programmer specifically did not implement? I suppose. But that's poor engineering.