$ cat <<EOF >c.c
> void f() {
>   int x;
>   if (x = 1) {}
> }
> EOF
$ gcc -Werror=parentheses -w -c -o /dev/null c.c
$ clang -Werror=parentheses -w -c -o /dev/null c.c
c.c:3:9: error: using the result of an assignment as a condition without parentheses [-Werror,-Wparentheses]
  if (x = 1) {}
      ~~^~~
c.c:3:9: note: place parentheses around the assignment to silence this warning
  if (x = 1) {}
        ^
      (    )
c.c:3:9: note: use '==' to turn this assignment into an equality comparison
  if (x = 1) {}
        ^
        ==
1 error generated.