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P3388R3 When Do You Know connect Doesn’t Throw? #8521

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P3388R3 When Do You Know connect Doesn’t Throw? #8521

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P3388R3 When Do You Know connect Doesn’t Throw?
eisenwave Nov 14, 2025
082ec3a
Fixup: add comma for readability
eisenwave Nov 15, 2025
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18 changes: 17 additions & 1 deletion source/exec.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -1185,7 +1185,8 @@
{ get_env(rcvr) } -> @\exposconcept{queryable}@;
} &&
@\libconcept{move_constructible}@<remove_cvref_t<Rcvr>> && // rvalues are movable, and
@\libconcept{constructible_from}@<remove_cvref_t<Rcvr>, Rcvr>; // lvalues are copyable
@\libconcept{constructible_from}@<remove_cvref_t<Rcvr>, Rcvr> && // lvalues are copyable
is_nothrow_move_constructible_v<remove_cvref_t<Rcvr>>;

template<class Signature, class Rcvr>
concept @\defexposconcept{valid-completion-for}@ = // \expos
Expand Down Expand Up @@ -2856,6 +2857,21 @@
Otherwise, \tcode{\exposid{connect-awaitable}(new_sndr, rcvr)}.
\end{itemize}
Except that \tcode{rcvr} is evaluated only once.
The program is ill-formed, no diagnostic required
if there exists an rvalue expression \tcode{rcvr2} such that:
\begin{itemize}
\item \tcode{decltype(rcvr2)} models \libconcept{receiver},
\item \tcode{noexcept(rcvr2)} is \tcode{true},
\item \tcode{is_same_v<decltype(get_env(rcvr2)), decltype(get_env(rcvr))>} is \tcode{true},
\item \tcode{noexcept(execution::connect(sndr, rcvr))} is \tcode{true}, and
\item \tcode{noexcept(execution::connect(sndr, rcvr2))}
is well-formed and evaluates to \tcode{false}.
\end{itemize}
\begin{note}
This allows determination of whether \tcode{connect} throws
with only the context of the environment,
such as within \tcode{get_completion_signatures}.
\end{note}

\mandates
The following are \tcode{true}:
Expand Down