`spawn_future` is not handling dependent senders correctly

[ericniebler]

In [exec.spawn.future]/p7, we see this:

7. Let spawn-future-state be the exposition-only class template:

namespace std::execution {
  template<class Alloc, scope_token Token, sender Sender, class Env>
  struct spawn-future-state                                                 // exposition only
    : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> {
    using sigs-t =                                                          // exposition only
      completion_signatures_of_t<future-spawned-sender<Sender, Env>>;

where spawn-future-sender<Sender, Env> is the type of write_env(stop-when(declval<Sender>(), declval<stoken-t>()), declval<Env>()).

The problem happens in the definition of sigs-t. There is a difference between asking for a sender's completion signatures with an empty environment, and asking with no environment.

In sigs-t, we are asking for the completion signatures with no environment. That is asking the sender for its non-dependent completions. But if Sender is a dependent sender, then so is future-spawned-sender<Sender, Env>, and so this line will not compile. A dependent sender cannot provide non-dependent completion signatures.

The spawn_future algorithm connects the future-spawned-sender with a receiver that has an empty environment. Therefore, we should be using the empty environment when computing the future-spawned-sender's completion signatures.

Proposed Resolution

Change [exec.spawn.future]/p7 as follows

namespace std::execution {
  template<class Alloc, scope_token Token, sender Sender, class Env>
  struct spawn-future-state                                                 // exposition only
-   : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> {
+   : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>, env<>>> {
    using sigs-t =                                                          // exposition only
-     completion_signatures_of_t<future-spawned-sender<Sender, Env>>;
+     completion_signatures_of_t<future-spawned-sender<Sender, Env>, env<>>;