Notes from the September, 2008 meeting:

This question is more involved than it might appear. For example, a constexpr member function is implicitly const; if the constexpr specifier is ignored, does that make the member function non-const? Also, should this provision apply only to dependent expressions in the function? Should it be an error if no constexpr function can be instantiated from the template, along the lines of the permission given in 13.8 [temp.res] paragraph 8 for an implementation to diagnose a template definition from which no valid specialization can be instantiated?