Additional note, June, 2014:

See issue 1941 for an alternative approach to this problem.

Notes from the April, 2013 meeting:

CWG agreed with the direction of defining such constructors as deleted.

[Adopted at the October, 2015 meeting as P0136R1.]

For an example like

  struct A {
    constexpr A(int, float = 0);
    explicit A(int, int = 0);
    A(int, int, int = 0) = delete;
  };

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

it is not clear from _N4527_.12.9 [class.inhctor] what should happen: is B::B(int) constexpr and/or explicit? Is B::B(int, int) explicit and/or deleted? Although the rationale given in the note in paragraph 7,

If two using-declarations declare inheriting constructors with the same signatures, the program is ill-formed (11.4 [class.mem], _N4868_.12.2 [over.load]), because an implicitly-declared constructor introduced by the first using-declaration is not a user-declared constructor and thus does not preclude another declaration of a constructor with the same signature by a subsequent using-declaration.

might be thought to apply, paragraph 1 talks about a set of candidate constructors based on their parameter types, so presumably such a set would contain only a single declaration of A::A(int) and one for A::A(int, int). The constructor characteristics of that declaration, however, are not specified.

One possibility might be to declare such a constructor, resulting from the transformation of more than one base class constrctor, to be deleted, so there would be an error only if it were used.