schedule_from isn't starting the schedule sender if decay-copying results throws

[ericniebler]

exec.schedule.from/p11 specifies shedule_from's completion operation as follows:

[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
    -> void {
  using result_t = decayed-tuple<Tag, Args...>;
  constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

  TRY-EVAL(rcvr, [&]() noexcept(nothrow) {
    state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
  }());

  if (state.async-result.valueless_by_exception())
    return;
  if (state.async-result.index() == 0)
    return;

  start(state.op-state);
}

so if emplacing the result tuple throws, set_error is immediately called on the downstream receiver. no attempt is made to post the completion to the specified scheduler. this is probably not the right behavior. the right thing, i think, is to catch the exception, emplace the exception_ptr into the async-result variant, and then start the schedule operation, as shown below:

[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept
    -> void {
  using result_t = decayed-tuple<Tag, Args...>;
  constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>;

  try {
    state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...);
  } catch(...) {
    if constexpr (nothrow)
      state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception());
    }
  }
  
  start(state.op-state);
}

we also need to change how we specify the variant type of state.async-result:

 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 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.
+env_of_t<Rcvr>>. Let as-tuple be an alias template such that
+as-tuple<Tag(Args...)> denotes the tuple specialization
+decayed-tuple<Tag, Args...>, and let is-nothrow-decay-copy-sig be
+a variable template such that is-nothrow-decay-copy-sig<Tag(Args...)>
+is a core constant expression of type bool const and whose value
+is true if the types Args... are all nothrow decay-copyable, and false
+otherwise. Let error-completion be a pack consisting of the type
+set_error_t(exception_ptr) if (is-nothrow-decay-copy-sig<Sigs> &&...)
+is false, and an empty pack otherwise. Then variant_t denotes the type
+variant<monostate, as-tuple<Sigs>..., error-completion...>, except
+with duplicate types removed.