--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
This is a consequence of the change Jonathan cites, to treat the inherited
constructor as a user-declared constructor that prevents the implicit
declaration of a default constructor in Derived.
The difference in access behavior follows from this; the standard says,
"A synonym created by a using-declaration has the usual accessibility for a
member-declaration. A using-declarator that names a constructor does not create
a synonym; instead, the additional constructors are accessible if they would be
accessible when used to construct an object of the corresponding base class,
and the accessibility of the using-declaration is ignored."
So if constructing 'd' calls the inherited constructor, it's ill-formed; if it
calls the implicitly-declared default constructor, it's well-formed.
I thought my change was an obvious bug fix, but it seems that other compilers
have the old semantics, so I should probably undo it.