The signatures came from another answer. I have no idea what the signatures are, or should be, or why there's a difference. My SFINAE code was to handle a sim

The signatures came from another answer. I have no idea what the signatures are, or should be, or why there's a difference. My SFINAE code was to handle a similar problem picking between a GNU-specific strerror_r and a POSIX strerror_r that have different signatures (both are available on Linux, determined by whether certain macros are set: https://linux.die.net/man/3/strerror_r https://linux.die.net/man/3/strerror_r , I couldn't just rely on those macros because I wanted my code to compile on any POSIX platform).