It's effectively catching the fact that `spectral_contrast.c` does not `#include "match.h"` — this is indeed directly a part of the underhandedness. The warnin

It's effectively catching the fact that `spectral_contrast.c` does not `#include "match.h"` — this is indeed directly a part of the underhandedness. The warning occurs because the function is not static — and therefore callable by other modules. Since it's missing a forward definition (a previous prototype), those modules must blindly declare the prototypes themselves… and their prototypes can get out of sync with the actual definition. That's what's happening here. `match.h` has the forward prototype, but it's very subtly different from the definition. Were match.h included, there'd be a much more glaring warning (or maybe even error).