I believe that the following is a bug. it is accepted by MSVC and comeau.

GCC seems to ADL-lookup names in namespaces that are only used in the template arguments of the function arguments, for example:

A<otherns::type> a;
adl_func(a); //otherns::adl_func is called

this can cause unintended conflicts with non-function names in "otherns", e.g. if the ADL function is called "apply" and the argument type happens to be a template instantiated with a type from namespace boost::mpl, only the fact that a header containing the type boost::mpl::apply is included can cause a compile error.

here's a simplified test case that causes a conflict:


namespace mpl{
  class apply{};
  class vector{};
}


template<class T>
void apply(T const &){}

template<class T>
class A{};

int main(){
  A<mpl::vector> a;
  apply(a);
}