Access to private static function during static member initialization

Question

I have a class with a static member. This will be initialized using a private static function of the same class.

#include <iostream>
#include <string>

class A
{
public:
    static std::string const s;

private:
    static std::string make()
    {
        return "S";
    }
};

std::string const A::s = A::make();

int main()
{
    std::cout << A::s << std::endl;
    // std::cout << A::make() << std::endl; // <-- Does not work
    return 0;
}

My question is: Because of which rule is this allowed? Clearly the commented part does not work, because I am not allowed to access a private function from outside the class. So why is the initialization of the private static member during startup a special case? (And on a side note: what is the intention of this rule? Is it to allow this exact case?)

I am aware of other mechanisms to initialize a static member (like here: Initializing private static members). But in my case the member is const, so as far as I know the only way to set it is via direct initalization at the place of definition.

Answer 1

Because the initialization of a static data member is considered part of the characterization of the class even though the static data member is defined at namespace scope (outside the class definition).

From the standard, class.static.data#note-1:

[Note 1: The initializer in the definition of a static data member is in the scope of its class ([basic.scope.class]). — end note]

[Example 1:

class process {
  static process* run_chain;
  static process* running;
};

process* process::running = get_main();
process* process::run_chain = running;

The definition of the static data member run_­chain of class process inhabits the global scope; the notation process​::​run_­chain indicates that the member run_­chain is a member of class process and in the scope of class process. In the static data member definition, the initializer expression refers to the static data member running of class process. — end example]