universal reference for template type error

Question

Excuse the bad title...

So, normally in a function like this:

template <class T>
void f(T&& i){
}

T&& is a universal reference. In a context like this it is an rvalue-reference:

template <class T>
struct s{

    void f(T&& t){}

};

This makes sense, as T is a fixed type relative to s of which f() is a method.

However, apparently in this context:

template <class T, unsigned n>
std::array<T, n>
f(T&&){
}

T&& is an rvalue-reference as well. This case is different from the above one though. So, what is the reason for T&& being an rvalue-reference here as well rather than a universal one?

Answer 1

That's because you supplied the parameter type explicitly (I am assuming this much, but this is the only way to make your example compile).

f<int,5> performs no type deductions at all, and its parameter is int&&. There is no reference collapsing going on.

You could work around this by writing the f template as such:

template<unsigned n, typename T>
array<decay_t<T>, n>
f(T&& t);

Now, t is a forwarding/universal reference if you don't provide the second template argument explicitly.