Ensure __decay builtin is not part of function signatures. (#1439)

---------

Co-authored-by: Eric Niebler <[email protected]>

Author: jehelset

include/stdexec/__detail/__type_traits.hpp

@@ -30,8 +30,18 @@ namespace stdexec {
   // __decay_t: An efficient implementation for std::decay
 #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 = __decay(_Ty);
+  using __decay_t = typename __tt::__decay_<sizeof(__tt::__wrap<_Ty>*) == ~0ul>::template __f<_Ty>;
 
 #elif STDEXEC_EDG()