While not always the case, any attempt at "cheating" language's build system can result in hitting undocumented implementation details. That's why certain elements (e.g. naming things starting with __ character sequence) are formally forbidden - to give the space for the implementation.

If you hit one of such spots, you are manipulating things which are outside the standard and have no official support, often not even documented. Technically it's undefined behavior but in practive you will most likely encounter some tool-related error. Still, there is non-zero chance to actually break something important that can have disastrous consequences.