Argument Dependent Name Lookup

What functions are found

Functions are found by first collecting a set of “associated classes” and “associated namespaces” that include one ore more of the following, depending on the argument type T. First, let us show the rules for classes, enumeration and class template specialization names.

If T is a nested class, member enumeration, then the surrounding class of it.
If T is an enumeration (it may also be a class member!), the innermost namespace of it.
If T is a class (it may also be nested!), all its base classes and the class itself. The innermost namespace of all associated classes.
If T is a ClassTemplate<TemplateArguments> (this is also a class!), the classes and namespaces associated with the template type arguments, the namespace of any template template argument and the surrounding class of any template template argument, if a template argument is a member template.

Now there are a few rules for builtin types as well

If T is a pointer to U or array of U, the classes and namespaces associated with U. Example: void (*fptr)(A); f(fptr);, includes the namespaces and classes associated with void(A) (see next rule).
If T is a function type, the classes and namespaces associated with parameter and return types. Example: void(A) would includes the namespaces and classes associated with A.
If T is a pointer to member, the classes and namespaces associated with the member type (may apply to both pointer to member functions and pointer to data member!). Example: B A::*p; void (A::*pf)(B); f(p); f(pf); includes the namespaces and classes associated with A, B, void(B) (which applies bullet above for function types).

All functions and templates within all associated namespaces are found by argument dependent lookup. In addition, namespace-scope friend functions declared in associated classes are found, which are normally not visible. Using directives are ignored, however.

All of the following example calls are valid, without qualifying f by the namespace name in the call.

namespace A {
   struct Z { };
   namespace I { void g(Z); }
   using namespace I;

   struct X { struct Y { }; friend void f(Y) { } };
   void f(X p) { }
   void f(std::shared_ptr<X> p) { }
}

// example calls
f(A::X());
f(A::X::Y());
f(std::make_shared<A::X>());

g(A::Z()); // invalid: "using namespace I;" is ignored!