[exec.then]

The names then, upon_error, and upon_stopped denote pipeable sender adaptor objects.

Let the expression then-cpo be one of then, upon_error, or upon_stopped.

For subexpressions sndr and f, if decltype(sndr)) does not satisfy sender, or decltype(f)) does not satisfy movable-value, then-cpo(sndr, f) is ill-formed.

Otherwise, the expression then-cpo(sndr, f) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(then-cpo, f, sndr)) except that sndr is evaluated only once.

For then, upon_error, and upon_stopped, let set-cpo be set_value, set_error, and set_stopped, respectively.

The exposition-only class template impls-for ([exec.snd.expos]) is specialized for then-cpo as follows:

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namespace std::execution { template<> struct impls-for<decayed-typeof<then-cpo>> : default-impls { static constexpr auto complete = []<class Tag, class. Args> (auto, auto& fn, auto& rcvr, Tag, Args&&. args) noexcept -> void { if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) { TRY-SET-VALUE(rcvr, invoke(std::move(fn), std::forward<Args>(args).); } else { Tag()(std::move(rcvr), std::forward<Args>(args).); } }; template<class Sndr, class. Env> static consteval void check-types(); }; }

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template<class Sndr, class. Env>
  static consteval void check-types();

Effects: Equivalent to: auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env).>(); auto fn = []<class. Ts>(decayed-typeof<set-cpo>(*)(Ts..) { if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts..>) throw unspecified-exception(); }; cs.for-each(overload-set{fn, [](auto){});

The expression then-cpo(sndr, f) has undefined behavior unless it returns a sender out_sndr that

(6.1)
invokes f or a copy of such with the value, error, or stopped result datums of sndr for then, upon_error, and upon_stopped, respectively, using the result value of f as out_sndr's value completion, and
(6.2)
forwards all other completion operations unchanged.