JuniorTheoryCommon

What does the program print?

C++

#include <iostream>

int main() {
    std::cout << (-1 < 0) << ' ' << (-1 < 0u) << '\n';
}

1 0

Expecting 1 1: in -1 < 0u the int is converted to unsigned first, and -1 becomes the largest unsigned value, which is not less than 0.
Expecting undefined behavior: converting a negative int to unsigned is well defined, and -1 always becomes the largest unsigned value.

Which warning flag makes GCC and Clang report a comparison of an int with an unsigned?
How does C++20's std::cmp_less(-1, 0u) compare the two values, and what does it return?

JuniorTheoryCommon

C++

#include <iostream>

int main() {
    std::cout << (-1 < 0) << ' ' << (-1 < 0u) << '\n';
}

1 0

Expecting 1 1: in -1 < 0u the int is converted to unsigned first, and -1 becomes the largest unsigned value, which is not less than 0.
Expecting undefined behavior: converting a negative int to unsigned is well defined, and -1 always becomes the largest unsigned value.

Which warning flag makes GCC and Clang report a comparison of an int with an unsigned?
How does C++20's std::cmp_less(-1, 0u) compare the two values, and what does it return?