> If you pass it some code in some third language which could parse as mal-formed A code, then how is to detect that?

This 'other language' approach doesn't strike me as a good way of thinking about it. It misses the point that C is pretty unique in its broad use of undefined behaviour. Unlike Java, where everything has an unambiguous definition. (Well, ignoring plenty of platform-specific variation points in the standard library, such as file-path syntax.)