diff --git a/include/gsl/pointers b/include/gsl/pointers
index f96d064..ae6f8c5 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -272,15 +272,13 @@ auto make_strict_not_null(T&& t) {
     return strict_not_null<std::remove_cv_t<std::remove_reference_t<T>>>{std::forward<T>(t)};
 }
 
-#if (defined(__cplusplus) && (__cplusplus >= 201703L))
+#if (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201907L))
 
 // deduction guides to prevent the ctad-maybe-unsupported warning
-template <class T>
-not_null(T ptr) -> not_null<T>;
-template <class T>
-strict_not_null(T ptr) -> strict_not_null<T>;
+template <class T> not_null(T) -> not_null<T>;
+template <class T> strict_not_null(T) -> strict_not_null<T>;
 
-#endif // (defined(__cplusplus) && (__cplusplus >= 201703L))
+#endif // (defined(__cpp_deduction_guides) && (__cpp_deduction_guides >= 201907L))
 
 } // namespace gsl