work around gcc bug

Author: ericniebler

include/exec/detail/basic_sequence.hpp

@@ -114,9 +114,9 @@ namespace exec = experimental::execution;
 namespace STDEXEC::__detail
 {
   template <auto _DescriptorFn>
-  extern __result_of<_DescriptorFn> __desc_of_v<exec::__seqexpr<_DescriptorFn>>;
+  extern decltype(_DescriptorFn()) __desc_of_v<exec::__seqexpr<_DescriptorFn>>;
 
   template <auto _DescriptorFn>
-  extern __declfn_t<__minvoke<__result_of<_DescriptorFn>, __q<exec::__basic_sequence_sender_t>>>
+  extern __declfn_t<__minvoke<decltype(_DescriptorFn()), __q<exec::__basic_sequence_sender_t>>>
     __demangle_v<exec::__seqexpr<_DescriptorFn>>;
 }  // namespace STDEXEC::__detail

include/stdexec/__detail/__basic_sender.hpp

@@ -447,7 +447,7 @@ namespace STDEXEC
     using __basic_sender_t = __basic_sender<_Tag, _Data, __demangle_t<_Child>...>::type;
 
     template <auto _Descriptor>
-    extern __declfn_t<__minvoke<__result_of<_Descriptor>, __q<__basic_sender_t>>>
+    extern __declfn_t<__minvoke<decltype(_Descriptor()), __q<__basic_sender_t>>>
       __demangle_v<__sexpr<_Descriptor>>;
   }  // namespace __detail
 }  // namespace STDEXEC