C++20 `format` function clashes with testlib's `format`

[quangloc99]

The new C++20 format is clashing with testlib's format one. These 2 are not compatible with each other, and they clash each other when there is using namespace std. I'm used to the testlib one when preparing the contest. However, the change in C++20 caught me off-guard, since std::format got resolved by gcc. To use testlib's format, I need to specify ::format.

I think there are 2 things that can be done. One is to warn about the new function, and two is to rename the function/scope it into a different namespace. I'm not sure about the warning part, since it seems not straight forward to do so. But this is a breaking part for a frequently used function.

[Cyril-44]

What you should do is stop using namespace std globally. Instead, you can specific using cin as using std::cin, etc. More formally, you should use std::cin. In many cases, your own name duplicates with some function names, and you either rename or use namespaces to specific them. That's why C++ introduces namespace.

Moreover, testlib.h is rebased from testlib.pas, so regarding of the compatibility, Mike use global namespaces. Such techs like this are very common, for example gmpxx.h uses global namespace for capability, although it's written for C++.