The case label values are converted to the promoted type of the expression being switched on, as if by a cast. In practice, this means discarding the high bits of a 32-bit value to produce a 16-bit one.
Code requiring this is dubious and would be a good candidate for a warning or a lint error, but it's allowed under the C standards.
The following code demonstrates the issue:
#include <stdio.h>
int main(void)
{
int i = 0x1234;
switch (i) {
case 0xABCD1234:
puts("good");
break;
default:
puts("bad");
}
}
