diff --git a/include/gsl/pointers b/include/gsl/pointers
index 42e8c09..2f1b15f 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -32,6 +32,15 @@
 namespace gsl
 {
 
+namespace details
+{
+template<typename T, typename = void>
+struct is_comparable_to_nullptr : std::false_type {};
+
+template <typename T>
+struct is_comparable_to_nullptr<T, std::enable_if_t<std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value>> : std::true_type {};
+}   // namespace details
+
 //
 // GSL.owner: ownership pointers
 //
@@ -68,8 +77,7 @@ template <class T>
 class not_null
 {
 public:
-    static_assert(std::is_convertible<decltype(std::declval<T>() != nullptr), bool>::value,
-                  "T cannot be compared to nullptr.");
+    static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
 
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(U&& u) : ptr_(std::forward<U>(u))