[Voted into the WP at the February, 2012 meeting; moved to DR at the October, 2012 meeting.]

Proposed resolution (August, 2011):

Change the indicated bullet of 7.7 [expr.const] paragraph 2 as follows:

• an invocation of an undefined constexpr function or an undefined constexpr constructor outside the definition of a constexpr function or a constexpr constructor;

The current wording appears to allow calling a constexpr function that is never defined within the body of a constexpr function. (The wording was intended to allow mutually-recursive constexpr functions but require that the not-yet-defined function be defined before it would be needed in an actual constant expression.)