message: das.named: Add `DefaultStore::default_for`.
diff: @@ -192,6 +192,7 @@ namespace das
  template <bool Effective, typename T>
  struct DefaultFor
  {
  + static constexpr bool has = false;
  template <typename Formal>
  DefaultFor(Formal const&)
  {}
  @@ -202,7 +203,8 @@ namespace das
  : public T::Formal::
  template Effective<typename T::Type, typename T::Type const&>
  {
  - using Type = typename T::Type;
  + static constexpr bool has = true;
  + using type = typename T::Type;
  using Super = typename T::Formal::
  template Effective<typename T::Type, typename T::Type const&>;
  using Super::Super;
  @@ -217,6 +219,9 @@ namespace das
  : DefaultFor<is_effective<Formal>::value, Formal>(
  std::forward<Args>(args))...
  {}
  +
  + template <typename T>
  + using default_for = DefaultFor<is_effective<T>::value, T>;
  };
  /*----------.
  