Should Writable require Semiregular, or "Move-Defaultable"

[CaseyCarter]

P0022R2 changes the Readable concept by replacing the Semiregular requirement with Movable and DefaultConstructible; this allows move-only types like unique_ptr<int> and optional<thread> to model Readable.

P0022R2 notably does not make the symmetric change to the Writable concept. Why should I be forbidden to write to these move-only types from which I may read?

Note that this discrepancy is exacerbated by the fact that both implementations (cmcstl2, range-v3) of the TS have already implemented the same relaxation for Writable.

EDIT:

Proposed Resolution

[Editorial note: Change the definition of Writable ([iterators.writable]) as follows: --end note]

template <class Out, class T>
concept bool Writable() {
-  return Semiregular<Out>() &&
+  return Movable<Out>() && DefaultConstructible<Out>() &&
    requires(Out o, T&& t) {
      *o = std::forward<T>(t); // not required to be equality preserving
    };
}

[ericniebler]

I think it was just an oversight.

[ericniebler]

Huh. Where do we currently require that iterators are copyable? Suggest: we add Semiregular to the Iterator concept. EDIT: We get that from WeaklyIncrementable.