How to refer to a doubly-templated free function in C++ - c++

Question

I have a templated class within which I define free functions taking references to that templated class. These free functions are also templated on a different parameter.
From outside the class I can call the free functions. However, I cannot find the correct syntax for one free function to call another.
Quick example:
template<typename T> class Foo {
template<typename S>
friend S f(const Foo &) { return S(); }
template<typename S>
friend S g(const Foo &s) {
return f(s); // See below, when instantiated, yields 'no matching function for call to f(const Foo &)'
}
};
float test1() {
Foo<int> o;
return f<float>(o); // Compiles
}
float test2() {
Foo<int> o;
return g<float>(o); // Fails to compile as line above errors
}
(c.f. this link too)
It seems by the point of the call to f(s) within g(), the outermost template has been lost. How might I re-specify the T in the call to f? I have checked on GCC4.7, 4.8, clang 3.2 all with equivalent errors.

Answer 1

When you call f(s) you need to specify the template parameter S because it can't be deduced from the argument s.
But if you change it to f<S>(s) (assuming you meant to call it with the same template argument S as g was called with) then you inhibit ADL, and the only way a friend function defined at class scope can be found is by ADL. So you need to add a declaration of f to the global namespace, so that the call in g can find it.
So to make it work you need to add these declarations before Foo
template<typename T> class Foo;
template<typename S, typename T>
S f(const Foo<T> &);
template<typename S, typename T>
S g(const Foo<T> &);
and change the call in g to be f<S>(s) or something else like f<x>(s)