[exec.connect] wording needs to ensure that in `connect(sndr, rcvr)` that `rcvr` expression is only evaluated once

[lewissbaker]

The current wording of connect(sndr, rcvr) defines the new_sndr expression as transform_sender(decltype(get-domain-late(sndr, get_env(rcvr))){}, sndr, get_env(rcvr)).

It then defines connect(sndr, rcvr) as expression equivalent to new_sndr.connect(rcvr).

As currently worded, this requires evaluating the rcvr expression twice. Note that the first usage in the new_sndr expression is unevaluated, but the second usage in get_env(rcvr) is evaluated.

I think we need to add an extra sentence at the end of this section saying "Where the expression rcvr is only evaluated once." or similar.

[ericniebler]

Proposed resolution

Change [exec.connect] p6 as follows:

 The expression connect(sndr, rcvr) is expression-equivalent to:
 
 - (6.1) new_sndr.connect(rcvr) if that expression is well-formed
 
     Mandates: The type of the expression above satisfies operation_state.
  
 - (6.2) Otherwise, connect-awaitable(new_sndr, rcvr)

+except that `rcvr` is evaluated only once.
+ 
 Mandates: sender<Sndr> && receiver<Rcvr> is true.

[ericniebler]

https://cplusplus.github.io/LWG/issue4208