Argument dependent lookup for classes in top-level namespace?

Question

I have a class that is in top-level namespace for historical reasons. I need to define a function for it that must¹ be found using argument-dependent lookup. Is the top-level namespace considered associated namespace of such class or do I have to make some workaround?

¹_{Otherwise the template that uses it might not find it, because symbols defined after the template are only seen when found by ADL.}

Answer 1

The global namespace is not special in this regard, it works just like any other namespace and is thus perfectly fine for your use case.

Live example 1 of global namespace working.

Live example 2 of ADL actually being the reason #1 works¹.

Here's the text of the live example:

#include <iostream>

struct X {};

template <class T>
void bar(T t)
{
  foo(t);
}

void foo(X x)
{
  std::cout << "Foo\n";
}

int main()
{
  X x;
  bar(x);
}

¹ This fails, because ADL cannot find the function, while normal lookup would. So it proves that #1 really works thanks to ADL.

Answer 2

Here is my take on the standard:

From [basic.lookup.argdep] (2):

— If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the innermost enclosing namespaces of its associated classes.

From [basic.namespace] (2) (and 3.3.6 where global namespace is defined):

The outermost declarative region of a translation unit is a namespace; see 3.3.6.

So I don't see anything to exclude the global namespace from ADL.