receiver/sender constraint that checks `get_env(rcvr)` is valid redundantly checks the result is `queryable`

[lewissbaker]

The concept for receiver and sender both check that get_env(o) is valid and returns a type that satisfies queryable. e.g.

// [exec.recv.concepts]
template<class Rcvr>
    concept receiver =
      derived_from<typename remove_cvref_t<Rcvr>::receiver_concept, receiver_t> &&
      requires(const remove_cvref_t<Rcvr>& rcvr) {
        { get_env(rcvr) } -> queryable;
      } &&
      move_constructible<remove_cvref_t<Rcvr>> &&       // rvalues are movable, and
      constructible_from<remove_cvref_t<Rcvr>, Rcvr>;   // lvalues are copyable

However, the get_env CPO itself already mandates that the result of the get_env call satisfies queryable, so the additional check in the receiver concept for this seems redundant.

Can we simplify the receiver concept to just check that get_env(rcvr) is valid?

Note that #321 is also exploring whether we need to check get_env(rcvr) at all, since it has an unconstrained fallback overload that should match any receiver.

[lewissbaker]

Further, the requires clause could be simplified from requires(const remove_cvref_t<Rcvr>& rcvr) to just requires(const Rcvr& rcvr) as reference-collapsing will result in the same parameter type.

And the derived_from constraint could be simplified to use remove_ref_t instead of remove_cvref_t when computing the receiver_concept type as the cv-qualification does not need to be stripped to get access to the ::receiver_concept member.

[lewissbaker]

I suggest simplifying the receiver concept to:

// [exec.recv.concepts]
template<class Rcvr>
    concept receiver =
      derived_from<typename remove_ref_t<Rcvr>::receiver_concept, receiver_t> &&
      requires(const Rcvr& rcvr) {
        get_env(rcvr);
      } &&
      move_constructible<remove_cvref_t<Rcvr>> &&       // rvalues are movable, and
      constructible_from<remove_cvref_t<Rcvr>, Rcvr>;   // lvalues are copyable

and similarly, simplifying the sender concept to:

  template<class Sndr>
    concept sender =
      bool(enable-sender<remove_cvref_t<Sndr>>) &&
      requires (const Sndr& sndr) {
        get_env(sndr);
      } &&
      move_constructible<remove_cvref_t<Sndr>> &&
      constructible_from<remove_cvref_t<Sndr>, Sndr>;