https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65300
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- [class.inhctor]/3: For each non-template constructor in the candidate set of inherited constructors other than a constructor having no parameters or a copy/move constructor having a single parameter, a constructor is implicitly declared with the same constructor characteristics unless there is a user-declared constructor with the same signature in the complete class where the using-declaration appears or the constructor would be a default, copy, or move constructor for that class. So inheriting constructors can't declare default, copy or move constructors, and the presence of another constructor in the class suppresses the implicit declaration of a default constructor.
