work around gcc-12 bug

Author: ericniebler

include/stdexec/__detail/__sender_concepts.hpp

@@ -54,9 +54,15 @@ namespace STDEXEC
                 && __std::move_constructible<__decay_t<_Sender>>
                 && __std::constructible_from<__decay_t<_Sender>, _Sender>;
 
+#if STDEXEC_GCC() && STDEXEC_GCC_VERSION < 1300
+  template <auto _Completions>
+  inline constexpr bool __constant_completion_signatures_v =
+    __valid_completion_signatures<std::remove_const_t<decltype(_Completions)>>;
+#else
   template <auto _Completions>
   inline constexpr bool __constant_completion_signatures_v =
     __valid_completion_signatures<decltype(_Completions)>;
+#endif
 
   template <class _Sender, class... _Env>
   concept sender_in =