Don't write `using namespace` at global scope in a header file

Reason

Doing so takes away an #includer's ability to effectively disambiguate and to use alternatives. It also makes #included headers order-dependent as they might have different meaning when included in different orders.

Example

// bad.h
#include <iostream>
using namespace std; // bad

// user.cpp
#include "bad.h"

bool copy(/*... some parameters ...*/);    // some function that happens to be named copy

int main()
{
    copy(/*...*/);    // now overloads local ::copy and std::copy, could be ambiguous
}

Note

An exception is using namespace std::literals;. This is necessary to use string literals in header files and given the rules - users are required to name their own UDLs operator""_x - they will not collide with the standard library.

Enforcement

Flag using namespace at global scope in a header file.