What’s the scope of inline friend functions?

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When you declare a friend function with an unqualified id in a class it names a function in the nearest enclosing namespace scope.

If that function hasn’t previously been declared then the friend declaration doesn’t make that function visible in that scope for normal lookup. It does make the declared function visible to argument-dependent lookup.

This is emphasised in many notes, but the definitive statement is in 7.3.1.2/3 (of ISO/IEC 14882:2011):

Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class or function the friend class or function is a member of the innermost enclosing namespace. The name of the friend is not found by unqualified lookup (3.4.1) or by qualified lookup (3.4.3) until a matching declaration is provided in that namespace scope (either before or after the class definition granting friendship). If a friend function is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (3.4.2). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace.

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