Clang has a -Wreserved-identifier warning, but that's only for reserved identifiers in the C standard, not for POSIX (e.g. names starting with double underscore or a single underscore followed by a capital letter).

The C standard has many more such adhoc reserved identifiers, for instance anything starting with `is`, `str` or `wcs` followed by a lowercase letter is technically a reserved identifier, hell, even any preprocessor macro starting with an `E` followed by a digit or uppercase letter is reserved.

Theoretically those reserved identifiers in the C standard matter more than _t because they affect all C code, not just C code targeting POSIX, yet nobody ever brings those up (because these are really only theoretical problems, and should they turn into actual problems one day they are trivial to fix).

It's important to differentiate between the C standard (e.g. what C compilers care about) and the POSIX standard (which C compilers do not care about), since not all C code runs on POSIX systems. It's only on some UNIXes where those two worlds overlap.

In the real-world, using _t typenames really is a complete non-issue.