message: Completely fulfil the Inspector concept
  When writing generic code and asking for Inspector::writes_state, we
  used to get a compiler error prior to this change.
diff: @@ -52,6 +52,7 @@ public:
  using result_type = void;
  static constexpr bool reads_state = true;
  + static constexpr bool writes_state = false;
  // -- constructors, destructors, and assignment operators --------------------
  