gsl::not_null: C.89: Make a hash noexcept (#1236)

Without this change, the guideline
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c89-make-a-hash-noexcept
would be violated.

The test fails before the changes here:

```
tests/pointers_tests.cpp:102:23: error: static assertion failed due to requirement 'noexcept(std::hash<gsl::not_null<std::shared_ptr<int>>>{}(std::declval()))': gsl::not_null hash operator must be noexcept
  102 |         static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tests/pointers_tests.cpp:108:23: error: static assertion failed due to requirement 'noexcept(std::hash<gsl::strict_not_null<std::shared_ptr<int>>>{}(std::declval()))': gsl::strict_not_null hash operator must be noexcept
  108 |         static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

Author: maflcko

include/gsl/pointers

@@ -235,7 +235,7 @@ not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;
 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()); }
+    std::size_t operator()(const T& value) const noexcept { return std::hash<U>{}(value.get()); }
 };
 
 template <class T, class U>

tests/pointers_tests.cpp

@@ -2,6 +2,7 @@
 
 #include <gsl/pointers>
 
+#include <functional>
 #include <memory>
 #include <type_traits>
 #include <utility>
@@ -98,4 +99,19 @@ TEST(pointers_test, member_types)
                   "check member type: element_type");
 }
 
+TEST(pointers_test, hash_noexcept_compiles)
+{
+    {
+        using Key = gsl::not_null<std::shared_ptr<int>>;
+        static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
+                      "gsl::not_null hash operator must be noexcept");
+    }
+
+    {
+        using Key = gsl::strict_not_null<std::shared_ptr<int>>;
+        static_assert(noexcept(std::hash<Key>{}(std::declval<Key>())),
+                      "gsl::strict_not_null hash operator must be noexcept");
+    }
+}
+
 } // namespace