gcc -Wparentheses warns for b=1; !a & b but not !a & 1

Question

Consider following short program:

int main() {
    int a = 5, b = 1, c;
    c = !a & b;
    c = !a & 1;

    return c;
}

Does anyone know why gcc gives a warning for b = 1; c = !a & b; but not for c = !a & 1;?

$gcc -Wall test.c
test.c: In function 'main':
test.c:3:9: warning: suggest parentheses around operand of '!' or change '&' to '&&' or '!' to '~' [-Wparentheses]
     c = !a & b;

My gcc version is 8.3.1 (I tried with one other version and the same thing happens).

I'm often surprised how 'smart' gcc is. And this one looks like a very easy problem to figure out. So why still a warning for the first one?

Answer 1

This warning comes into play when you have logical operators and bitwise operators in the same expression, as you would typically want either one or the other but not both.

The reason you don't see the warning in this case:

c = !a & 1;

Is because 1 is a boolean value and so both & and && will have the same effect in this expression.

If the constant were any value other than 1 or 0 it triggers the warning.