After P0547R0, const-qualified iterator types are not Readable or Writable

[ericniebler]

Readable requires Movable. const-qualified types are not Movable after P0547R0. Ouch.

Proposed Resolution

Accept the revised wording in P0547R1.

[ericniebler]

SUPERSEDED (...by incorporation into P0547R1)

Update section "Concept Readable" ([iterators.readable]) as follows (also includes part of the PR for #330):

template <class I>
concept bool Readable() {
- return Movable<I>() && DefaultConstructible<I>() &&
-   requires(const I& i) {
+ return requires {
    typename value_type_t<I>;
    typename reference_t<I>;
    typename rvalue_reference_t<I>;
-   { *i } -> Same<reference_t<I>>;
-   { ranges::iter_move(i) } -> Same<rvalue_reference_t<I>>;
  } &&
  CommonReference<reference_t<I>, value_type_t<I>&>() &&
  CommonReference<reference_t<I>, rvalue_reference_t<I>>() &&
  CommonReference<rvalue_reference_t<I>, const value_type_t<I>&>();
}

Update section "Concept Writable" ([iterators.writable]) as follows:

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

@CaseyCarter for your review.

[CaseyCarter]

The requires expression in Readable no longer needs any parameters. (There's also something funky with the indentation, but the Editor can fix that.)

[ericniebler]

Fixed by P0547, LWG wants to see the paper again.