What is the point of the complicated scoping rules for friend declarations?

Question

I recently discovered that friend declarations scoping follows extremely peculiar rules - if you have a friend declaration (definition) for a function or a class th

Answer 1

Well, for answering that, you have to look at another major feature of C++: Templates.

Consider a template such as this:

template 
struct magic {
    friend bool do_magic(T*) { return true; }
};

Used in code like this:

bool do_magic(void*) { return false; }

int main() {
    return do_magic((int*)0);
}

Will the exit-code be 0 or 1?

Well, it depends on whether magic was ever instantiated with int anywhere observable.
At least it would, if friend-functions only declared inline would be found by ordinary lookup-rules.
And you can't break that conundrum by just injecting everything possible, as templates can be specialized.

That was the case for a time, but was outlawed as "too magic", and "too ill-defined".

There were additional problems with name injection, as it wasn't nearly as well-defined as hoped for. See N0777: An Alternative to Name Injection from Templates for more.