Friend function in-class definition only allowed in non-local class definitions. What does it mean?

Question

Here (see point 2 description) I read that a friend function in-class definition is only allowed in non-local class definition.

What does that mean?

Answer 1

Non-local class is a class that's not local.

A local class is a class defined in a function - see paragraph "Local classes" on this page

"Friend function in-class definition" refers to ability to declare and define a non-member function inside a class, which is a friend of the class it's enclosed in.

class a_class
{
    friend std::ostream& operator<<(std::ostream& os, const a_class& x)
    {
        // ...
    }
};