Signed mod unsigned

Hi group,

today I ran into something I honestly did not expect. Consider this snippet:

#include <stdio.h>

int main() {
int i1, i2;
unsigned int u2;

i1 = -1;

i2 = 30;
u2 = 30;

printf("%d\n", i1 % i2);
printf("%d\n", i1 % u2);
return 0;
}

I expected to find the output "-1 / -1". Instead the result I got was
"-1 / 15". Apparently when getting the remainder of a signed by a
unsigned int, the signed is silently promoted to unsigned in twos
complement and then the operation is performed, i.e.

0xffffffff % 30 == 15

I would have expected at least a warning since I find this highly
unintuitive -- my expectation would be that the unsigned becomes a
(truncated) signed and a warning. Instead gcc does not emit a warning
(-Wall -Wextra) and silently performs above operation.

Is this really in accordance with the C standard or is gcc doing
something weird here?

Best regards,
Joe

--
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.

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