std::hash<gsl::not_null<T>> should really be constrained

[jwakely]

Ideally, std::hash<gsl::not_null<T>> would only be "enabled" (see [unord.hash]) if std::hash<T> is enabled.

This can be achieved very easily in C++20:

--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -175,6 +175,9 @@ not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 namespace std
 {
 template <class T>
+#if __cpp_concepts
+  requires std::is_invocable_v<std::hash<T>, T>
+#endif
 struct hash<gsl::not_null<T>>
 {
     std::size_t operator()(const gsl::not_null<T>& value) const { return hash<T>{}(value); }
@@ -278,6 +281,9 @@ template <class T> strict_not_null(T) -> strict_not_null<T>;
 namespace std
 {
 template <class T>
+#if __cpp_concepts
+  requires std::is_invocable_v<std::hash<T>, T>
+#endif
 struct hash<gsl::strict_not_null<T>>
 {
     std::size_t operator()(const gsl::strict_not_null<T>& value) const { return hash<T>{}(value); }

For portability to older standards something like this works:

--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -170,14 +170,27 @@ not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
 template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 
+template <class T, class U = decltype(std::declval<T>().get()), bool enabled = std::is_invocable_v<std::hash<U>, U>>
+struct not_null_hash
+{
+    std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }
+};
+
+template <class T, class U>
+struct not_null_hash<T, U, false>
+{
+  not_null_hash() = delete;
+  not_null_hash(const not_null_hash&) = delete;
+  not_null_hash& operator=(const not_null_hash&) = delete;
+};
+
 } // namespace gsl
 
 namespace std
 {
 template <class T>
-struct hash<gsl::not_null<T>>
+struct hash<gsl::not_null<T>> : gsl::not_null_hash<gsl::not_null<T>>
 {
-    std::size_t operator()(const gsl::not_null<T>& value) const { return hash<T>{}(value); }
 };
 
 } // namespace std
@@ -278,9 +291,8 @@ template <class T> strict_not_null(T) -> strict_not_null<T>;
 namespace std
 {
 template <class T>
-struct hash<gsl::strict_not_null<T>>
+struct hash<gsl::strict_not_null<T>> : gsl::not_null_hash<gsl::strict_not_null<T>>
 {
-    std::size_t operator()(const gsl::strict_not_null<T>& value) const { return hash<T>{}(value); }
 };
 
 } // namespace std