I'm going to go ahead and say that if (1) deciding whether code is valid for your language is undecidable, (2) no industrial compiler makes any effort to conser

I'm going to go ahead and say that if (1) deciding whether code is valid for your language is undecidable, (2) no industrial compiler makes any effort to conservatively accept only valid programs, and (3) unsoundness cannot even be detected at runtime (this is true of C in the multithreaded case) and may result in completely arbitrary behavior, your language is unsafe.