Additional note (December, 2021):

See duplicate issue 2493 for additional details.

Notes from the February, 2014 meeting:

CWG continued to express doubt as to the usefulness of this construct but felt that if it is permitted, the rules need clarification.

Additional note, November, 2013:

Doubt was expressed during the 2013-11-25 drafting review teleconference as to the usefulness of this provision. It is therefore being left open for further consideration after C++14 is finalized.

The current wording allows something like

  struct S {
    operator auto() { return 0; }
  } s;

If it is intended to be permitted, the details of its handling are not clear. Also, a similar syntax has been discussed as a possible future extension for dealing with proxy types in deduction which, if adopted, could cause confusion.