> B doesn't have a default constructor, just like A doesn't have one

B has an implicitly-declared default constructor, see
[class.default.ctor]/1.

Adding a user-declared constructor to B suppresses the implicit
declaration:

 #include <initializer_list>

 struct A { A(std::initializer_list<int>); };
 struct B : A {
  using A::A;
  B(void*);
 };

 A a {}; // ok
 B b {}; // ok