Bad practice to declare names in the standard namespace?

Question

I was looking through the Google C++ style guide, and came across this:

\"Do not declare anything in namespace std, not even forward declarations of standard library

Answer 1

Could someone explain what this means and why this is undefined behavior using example code?

The following program yields undefined behavior:

namespace std {
    void foo(int) { }
}

#include 

int main() {
    std::cout << "Hello World!" << std::endl;
}

Why? It declares a function named foo in namespace std. As for why this could cause a problem, consider: the Standard Library implementation might have its own function named foo() and that might be used by some component in . Your foo might then be a better match than the Standard Library implementation's foo.