Apply suggestions from code review

Author: ericniebler

include/stdexec/__detail/__type_traits.hpp

@@ -31,14 +31,17 @@ namespace stdexec {
 #if STDEXEC_HAS_BUILTIN(__decay)
 
   namespace __tt {
+    template <class>
+    struct __wrap;
+
     template <bool>
     struct __decay_ {
       template <class _Ty>
       using __f = __decay(_Ty);
     };
   } // namespace __tt
   template <class _Ty>
-  using __decay_t = typename __tt::__decay_<sizeof(__declval<_Ty>) == ~0ul>::template __f<_Ty>;
+  using __decay_t = typename __tt::__decay_<sizeof(__tt::__wrap<_Ty>*) == ~0ul>::template __f<_Ty>;
 
 #elif STDEXEC_NVHPC()