Given:

int main(void)
{
enum foo { A, B, C, D} bar;
        bar = D;
        if(bar == A) {
                ;
        } else if(bar == B) {
                ;
        } else {
                switch(bar) {
                        case C:
                        case D:
                        ;
                }
        }
        return 0;
}

barbeque/zarathustra:src: /usr/local/bin/gcc-4.1.0 -Wswitch-enum a.c
a.c: In function 'main':
a.c:10: warning: enumeration value 'A' not handled in switch
a.c:10: warning: enumeration value 'B' not handled in switch

A & B aren't valid targets for the switch statement by the time it's hit as they have been handled by the if statements above.

Locally built gcc:

barbeque/zarathustra:src: /usr/local/bin/gcc-4.1.0 -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.1.0/configure --prefix=/usr/local/stow/gcc-4.1.0 --enable-languages=c,c++,objc --infodir=/usr/local/stow/gcc-4.1.0/share/info --mandir=/usr/local/stow/gcc-4.1.0/share/man --enable-shared --enable-threads --enable-tls --program-suffix=-4.1.0 --enable-__cxa_atexit --enable-version-specific-runtime-libs --enable-targets=x86_64-unknown-linux-gnu --enable-bootstrap=lean --disable-biarch --disable-multilib
Thread model: posix
gcc version 4.1.0

On Debian/stable from amd64.debian.net.

Also happens on gcc 3.3.5 from the Debian apt archives.