P3388R3 When Do You Know connect Doesn’t Throw?

Fixes NB CA-334, FR-032-330, CA-329 (C++26 CD).

Author: eisenwave

source/exec.tex

@@ -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
@@ -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}: