[namespace.udecl] p4 says

If a constructor or assignment operator brought from a base class into a derived class has the signature of a copy/move constructor or assignment operator for the derived class ([class.copy.ctor], [class.copy.assign]), the using-declaration does not by itself suppress the implicit declaration of the derived class member;

According to [defns.signature.member]

⟨class member function⟩ name, parameter-type-list, class of which the function is a member, cv-qualifiers (if any), ref-qualifier (if any), and trailing requires-clause (if any)

A member of a base class cannot have the signature of the member of a derived class since the classes of which they are members are not the same.