I wonder why -Wsign-compare only warns when there is no int promotion?
No warning for this, where the result is “surprisingly” false because of int
promotion:
signed char i = (signed char) -3;
unsigned char j = (unsigned char) -3;
printf("i=%x j=%x i==j=%d\n", i, j, i==j);
gcc -Wsign-compare ~/test.c -o /tmp/glop && /tmp/glop
i=fffffffd j=fd i==j=0
But a warning for this where the result is true (which I believe represents a
lower risk than the above case):
signed int i = (signed int) -3;
unsigned int j = (unsigned int) -3;
printf("i=%x j=%x i==j=%d\n", i, j, i==j);
% gcc -Wsign-compare ~/test.c -o /tmp/glop && /tmp/glop
i=fffffffd j=fd i==j=0
/home/ddd/test.c:7:42: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
printf("i=%x j=%x i==j=%d\n", i, j, i==j);
^~
i=fffffffd j=fffffffd i==j=1
I did not find a rationale in the documentation, nor a good alternative flag
that would focus on comparisons with promotion.
Tested with several semi-old variants of GCC, e.g. 4.8.5, and more recent ones
e.g. 7.2.1, with consistent results.
gcc --version
gcc (GCC) 7.2.1 20170915 (Red Hat 7.2.1-2)
Thanks,
Christophe
