static void foo (char *s) { }
void bar (const char *s) { foo ((char *)s); }

gcc 4.2.0 produces this warning with -Wall -O2:
www.c:2: warning: passing argument 1 of `foo' discards qualifiers from pointer target type.

Due to the cast, the argument already has the right type, so gcc should not warn.
The warning disappears without -O2.
gcc 4.1.1 did not produce this warning.