LWG4202 enable-sender should be a variable template

Author: burblebee

source/exec.tex

@@ -1946,9 +1946,12 @@
       @\exposconcept{is-sender}@<Sndr> ||
       @\exposconcept{is-awaitable}@<Sndr, @\exposid{env-promise}@<env<>>>;                 // \ref{exec.awaitable}
 
+  template<class Sndr>
+    inline constexpr bool enable_sender = @\exposconcept{enable-sender}@<Sndr>;
+
   template<class Sndr>
     concept @\deflibconcept{sender}@ =
-      bool(@\exposconcept{enable-sender}@<remove_cvref_t<Sndr>>) &&
+      enable_sender<remove_cvref_t<Sndr>> &&
       requires (const remove_cvref_t<Sndr>& sndr) {
         { get_env(sndr) } -> @\exposconcept{queryable}@;
       } &&
@@ -1989,6 +1992,17 @@
 starting the resulting operation state
 are permissible completions for \tcode{Sndr} and \tcode{Env}.
 
+\pnum
+\remarks
+Pursuant to \ref{namespace.std},
+users may specialize \tcode{enable_sender} to
+\tcode{true} for cv-unqualified program-defined types that
+model \libconcept{sender}, and
+\tcode{false} for types that do not.
+Such specializations shall
+be usable in constant expressions\iref{expr.const} and
+have type \tcode{const bool}.
+
 \pnum
 A type models
 the exposition-only concept \defexposconcept{valid-completion-signatures}