C++'s complement of true is true, except when GCC says it's false

The complement of true is true, except when it's false

A standards proposal for bitmask operations on enums leads to a deep dive into C++ integer promotion quirks. Integral promotion forces small types up to int, and boolean conversion maps any nonzero value to true. So ~true becomes true. But GCC disagrees for enums backed by bool, truncating instead of converting, and unscoped enums can trigger undefined behavior when bitwise results fall outside their value range.