What is C++ name lookup doing here? (& is GCC right?)

Question

I was having a problem in some production code that I minimized to the following test case:

template<typename T>
void intermediate(T t)
{
    func(t); // line 4 ("func not declared in this scope")
}

namespace ns {
    struct type {};
}

void func(ns::type const & p); // line 11 ("declared here, later")

void foo(ns::type exit_node)
{
    intermediate(exit_node);  // line 15 ("required from here")
}

GCC 4.5 compiles this fine. Both with and without -std=c++11, 4.7 and 4.9 produce messages like:

test.cpp: In instantiation of ‘void intermediate(T) [with T = ns::type]’:
test.cpp:15:27:   required from here
test.cpp:4:5: error: ‘func’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
test.cpp:11:6: note: ‘void func(const ns::type&)’ declared here, later in the translation unit

All of the following three things will cause the file to successfully compile:

1. Move func(ns::type) into the ns namespace (allowing ADL to find it in ns)
2. Move type into the global namespace (allowing ADL to find it in ::)
3. Get rid of intermediate and call func directly from foo

So... what is going on here? Is it legal for GCC to reject this program? Why is func found by unqualified lookup in the third variant (call func directly from foo) but not found by unqualified lookup in the original variant at the point of instantiation?

Answer 1

The general rule is that anything that is not in the template definition context can only be picked up via ADL. In other words, normal unqualified lookup is performed only in the template definition context.

Since no declaration of func is visible when intermediate was defined, and func is not in a namespace associated with ns::type, the code is ill-formed.