[exec.sync.wait.var]

The name this_thread::sync_wait_with_variant denotes a customization point object.

For a subexpression sndr, let Sndr be decltype(into_variant(sndr)).

The expression this_thread::sync_wait_with_variant(sndr) is expression-equivalent to the following, except sndr is evaluated only once: apply_sender(get-domain-early(sndr), sync_wait_with_variant, sndr) Mandates:

(1.1)
sender_in<Sndr, sync-wait-env> is true.
(1.2)
The type sync-wait-with-variant-result-type<Sndr> is well-formed.
(1.3)
same_as<decltype(e), sync-wait-with-variant-result-type<Sndr>> is true, where e is the apply_sender expression above.

The expression sync_wait_with_variant.apply_sender(sndr) is equivalent to: using result_type = sync-wait-with-variant-result-type<Sndr>; if (auto opt_value = sync_wait(into_variant(sndr)) { return result_type(std::move(get<0>(*opt_value)); } return result_type(nullopt);

The behavior of this_thread::sync_wait_with_variant(sndr) is undefined unless:

(3.1)
It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes.

[Note 1:
The default implementation of sync_wait_with_variant achieves forward progress guarantee delegation by relying on the forward progress guarantee delegation provided by sync_wait.
— end note]
(3.2)
It returns the specified sender's async results as follows:
- (3.2.1)
  For a value completion, the result datums are returned in an engaged optional object that contains a variant of tuples.
- (3.2.2)
  For an error completion, an exception is thrown.
- (3.2.3)
  For a stopped completion, a disengaged optional object is returned.