E0137 expression must be a modifiable lvalue

int a, b;int main() {if (a < b || a = -1) {}return 0;}

错在将 == 写成了 =

这样相当于

int a, b;int main() {if (((a < b) || a) = -1) {}return 0;}

也就是在尝试给一个右值赋值