That message is only supposed to warn about compatibility to traditional
C, in case the function should ever be compiled without its prototype
being in effect. All other type checks are supposed to be in another
function, as documented, but that type check misses to report a few
error-prone type combinations (long to char, long to int).
30 years after the introduction of prototypes with C90, almost all
existing code uses prototypes. The warning has thus lost most of its
usefulness and can rather be confusing since a conversion from 'char' to
'long' is not problematic with prototypes in action, and the probability
of the code being backported to a pre-C90 compiler is diminishingly
small.
The words "due to prototype" now serve as a hint again. The proper fix
could be to suppress this warning in C99 mode since that's far enough
from traditional C.
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