int g;
int f1(int* p, int& r) { // regular_invocable
    ++g;
    ++*p;
    // r is not modified
    return 0;
}
int f2(int* p, int& r) { // not regular_invocable
    ++r;
    return 0;
}
struct F {
    mutable int m;
    int* p;
    int& r;
    int operator()() const { // regular_invocable
        ++m;
        ++*p;
        ++r;
        return 0;
    }
};