test.cpp

#include "bits/stdc++.h"
using namespace std;

int fn(int a, ...) {
    return 1;
}

double fn(int a, int b) {
    return 1.0;
}

void handle(int) {
    printf("...\n");
}

void handle(double) {
    printf("(int, int)\n");
}

int main() {
    // prints (int, int)
    handle(fn(10, 10));

    // prints (int, int)
    // I am not sure why, think it is because ... does not 'see' types
    handle(fn(10, 'a'));

    // prints ...
    handle(fn(10, 10, 10));
}