workaround for GCC

Author: ericniebler

include/stdexec/__detail/__concepts.hpp

@@ -97,6 +97,9 @@ namespace STDEXEC
   template <class _Ay, template <class...> class _Ty>
   concept __is_not_instance_of = !__is_instance_of<_Ay, _Ty>;
 
+  template <auto>
+  concept __constant = true;
+
   namespace __detail
   {
     template <auto>

include/stdexec/__detail/__get_completion_signatures.hpp

@@ -302,10 +302,18 @@ namespace STDEXEC
 
   ///////////////////////////////////////////////////////////////////////////////////////////////////
   // An minimally constrained alias for the result of get_completion_signatures:
+#if STDEXEC_GCC()
+  template <class _Sender, class... _Env>
+    requires enable_sender<__decay_t<_Sender>>
+            && __constant<STDEXEC::get_completion_signatures<_Sender, _Env...>()>
+  using __completion_signatures_of_t =
+    decltype(STDEXEC::get_completion_signatures<_Sender, _Env...>());
+#else
   template <class _Sender, class... _Env>
     requires enable_sender<__decay_t<_Sender>>
   using __completion_signatures_of_t =
     __mtypeof<STDEXEC::get_completion_signatures<_Sender, _Env...>()>;
+#endif
 
   ///////////////////////////////////////////////////////////////////////////////////////////////////
   // __get_child_completion_signatures