Rationale (October, 2015):

Without knowing the type of this, overload resolution cannot be performed, and it seems not worth the trouble to allow member functions only in the case where there is no overloading.

Bullet 13.3 of _N4567_.5.1.1 [expr.prim.general] permits only non-static data members to appear without an object expression in an unevaluated operand. There does not appear to be a good reason to exclude non-static member functions from this permission.