introduce gsl::not_null<T>::element_type (#1196)

* introduce gsl::not_null<T>::element_type

* use std::is_same instead of is_same_v

* fix: cannot put a non-pointer in a gsl::not_null

docs/headers.md

@@ -224,6 +224,14 @@ When a nullptr check fails, `std::terminate` is called.
 
 See [F.23: Use a `not_null<T>` to indicate that “null” is not a valid value](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-nullptr)
 
+#### Member Types
+
+```cpp
+using element_type = T;
+```
+
+The type of the pointed-to object.
+
 #### Member functions
 
 ##### Construct/Copy

include/gsl/pointers

@@ -98,6 +98,8 @@ class not_null
 public:
     static_assert(details::is_comparable_to_nullptr<T>::value, "T cannot be compared to nullptr.");
 
+    using element_type = T;
+
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(U&& u) noexcept(std::is_nothrow_move_constructible<T>::value) : ptr_(std::forward<U>(u))
     {

tests/pointers_tests.cpp

@@ -88,4 +88,10 @@ TEST(pointers_test, swap)
                   "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
 }
 
+TEST(pointers_test, member_types)
+{
+    static_assert(std::is_same<gsl::not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
+}
+
 } // namespace

tests/strict_notnull_tests.cpp

@@ -366,6 +366,13 @@ TEST(strict_notnull_tests, TestStrictNotNull)
     }
 }
 
+TEST(pointers_test, member_types)
+{
+    // make sure `element_type` is inherited from `gsl::not_null`
+    static_assert(std::is_same<gsl::strict_not_null<int*>::element_type, int*>::value,
+                  "check member type: element_type");
+}
+
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
 TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)