[over.match.viable]:

A candidate function having more than m parameters is viable only if the (m+1)^st parameter has a default argument ([dcl.fct.default]).¹²⁴

and footnote 124 says:

According to [dcl.fct.default], parameters following the (m+1)^st parameter must also have default arguments.

However, according to [dcl.fct.default], subsequent parameters do not need to have default arguments if they were expanded from a parameter pack.

template<class ... T> struct C {
  void f(int n = 0, T...);
};
C<int> c;                       // OK, instantiates declaration void C​::​f(int n = 0, int)