Rationale (November, 2016):

Such constructors can be useful for guaranteeing static initialization of namespace-scope objects.

Given an example like:

  struct S {
    constexpr S(): i(42) {  }
    ~S();
    int i;
  };
  double x[S().i];  // Error

such a constexpr constructor is completely useless, but there doesn't appear to be anything in the current wording making it ill-formed. Should it be?