$ cat a.c
int main() {
    double x = 1;
    double y = 1;
    int z = x & y;
    return 0;
}

$ gcc -Wall -Wextra -pedantic a.c -o a && a
a.c: In function ‘main’:
a.c:4:15: error: invalid operands to binary & (have ‘double’ and ‘double’)
     int z = x & y;
               ^
a.c:4:9: warning: unused variable ‘z’ [-Wunused-variable]
     int z = x & y;
         ^