P3570R2 optional variants in sender/receiver

Fixes #7955
Fixes cplusplus/papers#2249

Author: AlisdairM

source/exec.tex

@@ -464,6 +464,7 @@
   struct @\libglobal{get_forward_progress_guarantee_t}@ { @\unspec@ };
   template<class CPO>
     struct @\libglobal{get_completion_scheduler_t}@ { @\unspec@ };
+  struct get_await_completion_adaptor_t { @\unspec@ };
 
   inline constexpr get_domain_t @\libglobal{get_domain}@{};
   inline constexpr get_scheduler_t @\libglobal{get_scheduler}@{};
@@ -472,6 +473,7 @@
   inline constexpr get_forward_progress_guarantee_t @\libglobal{get_forward_progress_guarantee}@{};
   template<class CPO>
     constexpr get_completion_scheduler_t<CPO> @\libglobal{get_completion_scheduler}@{};
+  inline constexpr get_await_completion_adaptor_t get_await_completion_adaptor{};
 
   struct @\libglobal{get_env_t}@ { @\unspec@ };
   inline constexpr get_env_t @\libglobal{get_env}@{};
@@ -1005,6 +1007,22 @@
 \tcode{forwarding_query(get_completion_scheduler<\exposid{completion-tag}>)}
 is a core constant expression and has value \tcode{true}.
 
+\rSec2[exec.get.await.adapt]{\tcode{execution::get_await_completion_adaptor}}
+
+\pnum
+\tcode{get_await_completion_adaptor} asks a queryable object for
+its associated awaitable completion adaptor.
+
+\pnum
+The name \tcode{get_await_completion_adaptor} denotes a query object.
+For a subexpression \tcode{env},
+\tcode{get_await_completion_adaptor(env)} is expression-equivalent to
+\tcode{\exposid{MANDATE-NOTHROW}(\exposid{AS-CONST}(env).query(get_await_completion_adaptor))}.
+
+\pnum
+\tcode{forwarding_query(execution::get_await_completion_adaptor)}
+is a core constant expression and has value \tcode{true}.
+
 \rSec1[exec.sched]{Schedulers}
 
 \pnum
@@ -5442,6 +5460,13 @@
         { p.unhandled_stopped() } -> @\libconcept{convertible_to}@<coroutine_handle<>>;
       };
 
+  template<class Sndr>
+    concept @\defexposconcept{has-queryable-await-completion-adaptor}@              // \expos
+      = @\libconcept{sender}@<Sndr>
+        && requires(Sndr&& sender) {
+        get_await_completion_adaptor(get_env(sender));
+      };
+
   template<class Sndr, class Promise>
     class @\exposidnc{sender-awaitable}@;                                     // \expos
 }
@@ -5583,6 +5608,13 @@
 in a coroutine with promise type \tcode{U} is expression-equivalent to
 the same expression in a coroutine with promise type \tcode{Promise}.
 \item
+Otherwise, \tcode{\exposid{sender-awaitable}\{\exposid{adapted-expr}, p\}}
+if \tcode{\exposid{has-queryable-await-completion-adaptor}<Expr>}
+and \tcode{\exposid{awaitable-sender}<decltype((\exposid{adapted-expr})), Promise>}
+are both satisfied, where \exposid{adapted-expr} is
+\tcode{get_await_completion_adaptor(get_env(expr))(expr)},
+except that \tcode{expr} is evaluated only once.
+\item
 Otherwise, \tcode{\exposid{sender-awaitable}\{expr, p\}}
 if \tcode{\exposconcept{awaitable-sender}<Expr, Promise>} is \tcode{true}.
 \item

source/support.tex

@@ -799,7 +799,7 @@
 #define @\defnlibxname{cpp_lib_saturation_arithmetic}@             202311L // also in \libheader{numeric}
 #define @\defnlibxname{cpp_lib_scoped_lock}@                       201703L // also in \libheader{mutex}
 #define @\defnlibxname{cpp_lib_semaphore}@                         201907L // also in \libheader{semaphore}
-#define @\defnlibxname{cpp_lib_senders}@                           202406L // also in \libheader{execution}
+#define @\defnlibxname{cpp_lib_senders}@                           202506L // also in \libheader{execution}
 #define @\defnlibxname{cpp_lib_shared_mutex}@                      201505L // also in \libheader{shared_mutex}
 #define @\defnlibxname{cpp_lib_shared_ptr_arrays}@                 201707L // also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_shared_ptr_weak_type}@              201606L // also in \libheader{memory}