constraints on schedule_from's get-state function are incorrect

[ericniebler]

[exec.schedule.from] p6 reads:

The member impls-for<schedule_from_t>​::​get-state is initialized with a callable object equivalent to the following lambda:

[]<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
   requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> {

The constraint should account for the fact that the child sender will be connected with FWD-ENV(get_env(rcvr)).

Proposed Resolution

Change [exec.snd.expos] p2 as follows (underlined text is new):

2. For a queryable object env, FWD-ENV(env) is an expression whose
   type satisfies queryable such that for a query object q and a pack
   of subexpressions as, the expression FWD-ENV(env).query(q, as...)
   is ill-formed if forwarding_query(q) is false; otherwise, it is
   expression-equivalent to env.query(q, as...).
   The type FWD-ENV-T(Env) is decltype(FWD-ENV(declval<Env>())).

Change [exec.schedule.from] p6 as follows:

The member `impls-for<schedule_from_t>​::​get-state` is initialized with
a callable object equivalent to the following lambda:

  []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below)
-   requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> {
+   requires sender_in<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)> {

Change [exec.schedule.from] p8 as follows:

Let Sigs be a pack of the arguments to the completion_signatures
specialization named by
-completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>. Let
+completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>. Let
as-tuple be an alias template that transforms a completion signature
Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>.
Then variant_t denotes the type variant<monostate, as-tuple<Sigs>...>,
except with duplicate types removed.