message: das.named: simplify
  One less structure to instantiate.
diff: @@ -33,6 +33,8 @@ namespace elle
  typename T::Formal
  _make_formal(int);
  + /// T::Formal if it exists, otherwise T, both with references
  + /// stripped.
  template <typename T>
  using make_formal =
  std::remove_reference_t<decltype(_make_formal<T>(42))>;
  @@ -82,14 +84,9 @@ namespace elle
  | DefaultStore |
  `-------------*/
  - template <typename T>
  - struct is_effective
  - {
  - static constexpr bool value =
  - !std::is_same<decltype(_make_formal<T>(42)), T>::value;
  - };
  -
  - template <typename Formal, bool Effective = is_effective<Formal>::value>
  + template <typename Formal,
  + bool IsEffective = !std::is_same<decltype(_make_formal<Formal>(42)),
  + Formal>::value>
  struct DefaultFor;
  template <typename T>
  