34 Execution control library [exec]

stopped_as_optional maps a sender's stopped completion operation into a value completion operation as an disengaged optional.

The sender's value completion operation is also converted into an optional.

The result is a sender that never completes with stopped, reporting cancellation by completing with an disengaged optional.

The name stopped_as_optional denotes a pipeable sender adaptor object.

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

The expression stopped_as_optional(sndr) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(stopped_as_optional, {}, sndr)) except that sndr is only evaluated once.

Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)).

If sender-for<Sndr, stopped_as_optional_t> is false, or if the type single-sender-value-type<Sndr, Env> is ill-formed or void, then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: auto&& [_, _, child] = sndr; using V = single-sender-value-type<Sndr, Env>; return let_stopped( then(std::forward_like<Sndr>(child), []<class... Ts>(Ts&&... ts) noexcept(is_nothrow_constructible_v<V, Ts...>) { return optional<V>(in_place, std::forward<Ts>(ts)...); }), []() noexcept { return just(optional<V>()); });