[Moved to DR at the October, 2015 meeting.]

Proposed resolution (May, 2015):

Change 13.10.4 [temp.over] paragraph 1 as follows,

...If, for a given function template, argument deduction fails or the synthesized function template specialization would be ill-formed, no such function is added to the set of candidate functions for that template.

In an example like

  struct A { operator int(); };
  template<typename T> T operator<<(T, int);
  void f(A a) { 1 << a; }

Template argument deduction succeeds for the operator template, producing the signature operator<<(int,int). The resulting declaration is synthesized and added to the overload set, per 13.10.4 [temp.over] paragraph 1. However, this violates the requirement of 12.4 [over.oper] paragraph 6,

An operator function shall either be a non-static member function or be a non-member function that has at least one parameter whose type is a class, a reference to a class, an enumeration, or a reference to an enumeration.

This is not a SFINAE context, so the program is ill-formed, rather than selecting the built-in operator.