Since [over.match.general] p2 says

Each of these contexts defines the set of candidate functions and the list of arguments in its own unique way.

However, [over.match.ctor] is not clear for the argument list of the default-initialization context. [over.match.ctor] only says

The argument list is the expression-list or assignment-expression of the initializer.

Neither expression-list nor assignment-expression can be empty. We should explicitly specify the list for default-initialization. Append that

or empty for default-initialization.

This can clarify [over.match.viable] p2

First, to be a viable function, a candidate function shall have enough parameters to agree in number with the arguments in the list.

• [...]
• A candidate function having more than m parameters is viable only if all parameters following the
  m^th have default arguments ([dcl.fct.default]). For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.

And the relative example in [over.match.best.general] 2.7

struct A {
  A(int = 0);
};

struct B: A {
  using A::A;
  B();
};

int main() {
  B b;              // OK, B​::​B()
}

we expect the number of the arguments in the list is zero in the default-initialization, therefore A::A(int) is considered to have zero parameters for the purpose of the overload resolution as per [over.match.viable] p2.