Fix std::hash compilation error for gsl::not_null<std::shared_ptr<T>> (#1210)

* Initial plan for issue

* Fix hash compilation issue for gsl::not_null with shared_ptr

Co-authored-by: carsonRadtke <[email protected]>

* Add comment explaining safe usage of typename T::element_type in not_null_hash

Co-authored-by: carsonRadtke <[email protected]>

* Initial plan for issue

* Remove upgrade_checklist.md file as requested

Co-authored-by: carsonRadtke <[email protected]>

* restore docs/upgrade_checklist.md

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: carsonRadtke <[email protected]>
Co-authored-by: Carson Radtke <[email protected]>

Author: Copilot

include/gsl/pointers

@@ -231,7 +231,8 @@ template <class T>
 not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 
 
-template <class T, class U = decltype(std::declval<const T&>().get()), bool = std::is_default_constructible<std::hash<U>>::value>
+// T is conceptually a pointer so we don't have to worry about it being a reference and violating std::hash requirements
+template <class T, class U = typename T::element_type, bool = std::is_default_constructible<std::hash<U>>::value>
 struct not_null_hash
 {
     std::size_t operator()(const T& value) const { return std::hash<U>{}(value.get()); }

tests/notnull_tests.cpp

@@ -735,4 +735,18 @@ TEST(notnull_tests, TestStdHash)
         EXPECT_FALSE(hash_nn(nn) == hash_intptr(&y));
         EXPECT_FALSE(hash_nn(nn) == hash_intptr(nullptr));
     }
+
+    {
+        auto x = std::make_shared<int>(42);
+        auto y = std::make_shared<int>(99);
+        not_null<std::shared_ptr<int>> nn{x};
+        const not_null<std::shared_ptr<int>> cnn{x};
+
+        std::hash<not_null<std::shared_ptr<int>>> hash_nn;
+        std::hash<std::shared_ptr<int>> hash_sharedptr;
+
+        EXPECT_TRUE(hash_nn(nn) == hash_sharedptr(x));
+        EXPECT_FALSE(hash_nn(nn) == hash_sharedptr(y));
+        EXPECT_TRUE(hash_nn(cnn) == hash_sharedptr(x));
+    }
 }