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); } -- Summary: ADL names are looked up in namespaces that are only referenced by template arguments Product: gcc Version: 4.3.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: sstrasser at systemhaus-gruppe dot de http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44320