Add constexpr to not_null comparison operators

Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

Author: Copilot

include/gsl/pointers

@@ -173,47 +173,47 @@ std::ostream& operator<<(std::ostream& os, const not_null<T>& val)
 #endif // !defined(GSL_NO_IOSTREAMS)
 
 template <class T, class U>
-auto operator==(const not_null<T>& lhs,
+constexpr auto operator==(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
     -> decltype(lhs.get() == rhs.get())
 {
     return lhs.get() == rhs.get();
 }
 
 template <class T, class U>
-auto operator!=(const not_null<T>& lhs,
+constexpr auto operator!=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
     -> decltype(lhs.get() != rhs.get())
 {
     return lhs.get() != rhs.get();
 }
 
 template <class T, class U>
-auto operator<(const not_null<T>& lhs,
+constexpr auto operator<(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(std::less<>{}(lhs.get(), rhs.get())))
     -> decltype(std::less<>{}(lhs.get(), rhs.get()))
 {
     return std::less<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator<=(const not_null<T>& lhs,
+constexpr auto operator<=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(std::less_equal<>{}(lhs.get(), rhs.get())))
     -> decltype(std::less_equal<>{}(lhs.get(), rhs.get()))
 {
     return std::less_equal<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator>(const not_null<T>& lhs,
+constexpr auto operator>(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(std::greater<>{}(lhs.get(), rhs.get())))
     -> decltype(std::greater<>{}(lhs.get(), rhs.get()))
 {
     return std::greater<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
-auto operator>=(const not_null<T>& lhs,
+constexpr auto operator>=(const not_null<T>& lhs,
                 const not_null<U>& rhs) noexcept(noexcept(std::greater_equal<>{}(lhs.get(), rhs.get())))
     -> decltype(std::greater_equal<>{}(lhs.get(), rhs.get()))
 {

tests/CMakeLists.txt

@@ -204,6 +204,7 @@ add_executable(gsl_tests
     assertion_tests.cpp
     at_tests.cpp
     byte_tests.cpp
+    constexpr_notnull_tests.cpp
     notnull_tests.cpp
     owner_tests.cpp
     pointers_tests.cpp