Condider this

/* bar.c */

void bar(char a)
{
   char b = a;
}

void frob(void)
{
   bar('a');
}

Due to the automatic promotion of char to int in function calls [same would apply to short or C99 _Bool], then this causes a warning with -Wconversion, thus:

gcc -Wconversion -c bar.c
bar.c: In function 'frob':
bar.c:10: warning: passing argument 1 of 'bar' with different width due to prototype

This greatly limits the usefulness of -Wconversion.

As a user I either have to
a) put up with many thousands of useless warnings
b) not use -Wconversion, or use it with heavy filtering
c) never pass char/short/bool arguments in functions

A similar thing happens with float/double:
bar.c:10: warning: passing argument 1 of 'bar' as 'float' rather than 'double' due to prototype

This only happens with gcc, not g++.