false negative unsigned integer overflow when wrapping around in different `if` branches

[carenas]

The following code:

int main(void)
{
	unsigned i = 1;
	int c = 1;

	if (c) {
		while (i-- > 0) { }
	} else {
		return i;
	}
	return 128;
}

shows

$ clang -fsanitize=integer t.c
$ ./a.out; echo $?
t.c:9:11: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior t.c:9:11 
128

note that while the variable is being used in the else branch, the execution through the main branch where it wraparounds, prevents it to reaching that path, and therefore even if technically DID wraparound, reporting that is not useful.

Additionally, it doesn't trigger if the variable is signed (which is actually UB), so there might be a fix in that implementation which might be missing for unsigned.