Partial specialization of variadic templates

14.8.2.4, section 11 (I refer to draft N3242).

In most cases, all template parameters must have values in order for
deduction to succeed, but for partial ordering purposes a template
parameter may remain without a value provided it is not used in the
types being used for partial ordering. [ Note: A template parameter
used in a non-deduced context is considered used. —end note ] [
Example:

template <class T> T f(int); // #1
template <class T, class U> T f(U); // #2
void g() {
f<int>(1); // calls #1
}

In your case, #3 will be used.