P3433R1 Allocator Support for Operation States

Fixes #7957
Fixes cplusplus/papers#2098

Author: AlisdairM

source/exec.tex

@@ -1819,7 +1819,7 @@
 \begin{codeblock}
 []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> decltype(auto) {
   auto& [_, data, ...child] = sndr;
-  return std::forward_like<Sndr>(data);
+  return @\exposid{allocator-aware-forward}@(std::forward_like<Sndr>(data), rcvr);
 }
 \end{codeblock}
 
@@ -1916,6 +1916,36 @@
 \end{itemize}
 \end{itemdescr}
 
+\begin{itemdecl}
+template<class T, class Context>
+decltype(auto) @\exposid{allocator-aware-forward}@(T&& obj, Context&& context); // \expos
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\exposid{allocator-aware-forward} is an exposition-only function used to
+either create a new object of type \tcode{T} from \tcode{obj}
+or forward \tcode{obj} depending on whether an allocator is available.
+If the environment associated with context provides an allocator
+(i.e., the expression \tcode{get_allocator(get_env(context))} is valid,
+let \exposid{alloc} be the result of this expression
+and \tcode{P} be \tcode{remove_cvref_t<T>}.
+
+\pnum
+\returns
+\begin{itemize}
+\item
+If \exposid{alloc} is not defined, returns \tcode{std::forward<T>(obj)},
+\item
+otherwise if \tcode{P} is a specialization of \exposid{product-type},
+returns an object of type \tcode{P} whose elements are initialized using
+\tcode{make_obj_using_allocator<decltype(e)> (std::forward_like<T>(e), \exposid{alloc})}
+where \tcode{e} is the corresponding element of \tcode{obj},
+\item
+otherwise, returns \tcode{make_obj_using_allocator<P>(std::forward<T>(obj), \exposid{alloc})}.
+\end{itemize}
+\end{itemdescr}
+
 \rSec2[exec.snd.concepts]{Sender concepts}
 
 \pnum
@@ -3515,7 +3545,8 @@
     args_variant_t @\exposid{args}@;        // \expos
     ops2_variant_t @\exposid{ops2}@;        // \expos
   };
-  return @\exposid{state-type}@{std::forward_like<Sndr>(fn), @\exposid{let-env}@(child), {}, {}};
+  return @\exposid{state-type}@{@\exposid{allocator-aware-forward}@(std::forward_like<Sndr>(fn), rcvr),
+                    @\exposid{let-env}@(child), {}, {}};
 }
 \end{codeblock}