[C++][NOTE][BEGINNER] Ambiguous conversion

This overload is ambiguous.

There are two viable functions for the call f(-2.5).

For the compiler to select one, one of them needs to be better than the other,

or the program is ill-formed. In our case, they are equally good, making the program ill-formed.

According to §13.3.3 in the standard, a viable one-argument function is better than another

if the conversion sequence for the argument is better. So why isn't the int conversion

sequence better than the unsigned conversion sequence, given that the double is signed?

All conversions are given a rank, and both "double => int" and "double => unsigned int"

are of type "floating-integral conversion", which has rank "conversion". See Table 12

in the standard and §4.9. Since they have the same rank, no conversion is better than the other,

and the program is ill-formed.

#include <iostream>

void f(int) { std::cout << 1; }
void f(unsigned) { std::cout << 2; }

int main() {
  f(-2.5);
}