Proposed resolution (November, 2018):

Change 9.4.2 [dcl.init.aggr] paragraph 12 as follows:

If a reference member is initialized from its has a default member initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that default member initializer, the program is ill-formed. [Example:

  struct A;
  extern A a;
  struct A {
    const A& a1 { A{a,a} }; // OK
    const A& a2 { A{} };    // error
  };
  A a{a,a};                 // OK

  struct B {
  int n = B{}.n;            // error
  };

—end example]

Notes from the March, 2018 meeting:

There was a suggestion that creating an object of the containing class in a default member initializer should be prohibited. That would presumably be a difference between the reference member and non-reference member cases, since the intent is to allow creation of a temporary for a reference member to bind to. The suggested approach for drafting was simply to remove the restriction to references in 9.4.2 [dcl.init.aggr] paragraph 11.

[Accepted as a DR at the February, 2019 meeting.]

Consider an example like:

  struct A {
    int n = A{}.n;
  };

There doesn't seem to be a good reason to support this kind of thing, and it would be simpler to say that a default member initializer can't trigger any direct or indirect use of itself in general, rather than just the two special cases that were banned by issues 1696 and 1397.