[basic.def.odr] (6.2.1.3)
Coming from: http://stackoverflow.com/questions/41133290 where T.C. asserted that this should be "not odr-used within the definition of D." Otherwise another entity odr-using the constant could break existing definitions non-odr-use. Alternatively this could read "is not odr-used in all definitions of D" but that language seems more ambiguous.

The example code:

static constexpr int CONSTANT = 2;
inline int f() {
  return CONSTANT;
}

If this appears in a header, it could be broken later (by a strict reading of the current language) if anyone, anywhere odr-uses CONSTANT