why no need of forward declaration in static dispatching via templates?

Question

I am playing a bit with static polymorphism, I\'m calling a function which internally calls the \"right\" specialized function depending on the type of the initial argument (bas

Answer 1

f_helper(typename T::tag_type{}) is a type-dependent expression because T::tag_type is a dependent type. This means that f_helper doesn't need to be visible until f is instantiated due to two phase lookup.

EDIT: I'm pretty sure that this is actually undefined behaviour. If we look at 14.6.4.2 [temp.dep.candidate] we see this passage:

For a function call that depends on a template parameter, the candidate functions are found using the usual lookup rules (3.4.1, 3.4.2, 3.4.3) except that:

— For the part of the lookup using unqualified name lookup (3.4.1) or qualified name lookup (3.4.3), only function declarations from the template definition context are found.

— For the part of the lookup using associated namespaces (3.4.2), only function declarations found in either the template definition context or the template instantiation context are found.

If the function name is an unqualified-id and the call would be ill-formed or would find a better match had the lookup within the associated namespaces considered all the function declarations with external linkage introduced in those namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts, then the program has undefined behavior.

The last paragraph to me indicates this is undefined behaviour. The function call that depends on a template parameter here is f_helper(typename T::tag_type{}). f_helper isn't visible when f is instantiated, but it would be if we performed name lookup after all translation units have been compiled.