From 5dc7fae1199e90b875decfae7b91e83d27881c81 Mon Sep 17 00:00:00 2001
From: Dmitry Kobets <89153909+dmitrykobets-msft@users.noreply.github.com>
Date: Tue, 9 May 2023 09:06:53 -0700
Subject: [PATCH] Use the implementation-defined strict total order for pointer
 comparisons with `not_null` (#1106)

Using `<`,`<=`,`>`,`>=` to compare unrelated pointers gives an unspecified result according to the standard.
This PR replaces the usage of these operators in `gsl::not_null` with the STL counterparts, which would leverage any implementation-defined strict total ordering for pointers.

Resolves #880
---
 include/gsl/pointers | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/gsl/pointers b/include/gsl/pointers
index 5cf1cf7..9c65a96 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -175,34 +175,34 @@ auto operator!=(const not_null<T>& lhs,
 
 template <class T, class U>
 auto operator<(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(lhs.get() < rhs.get()))
-    -> decltype(lhs.get() < rhs.get())
+               const not_null<U>& rhs) noexcept(noexcept(std::less<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::less<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() < rhs.get();
+    return std::less<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator<=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() <= rhs.get()))
-    -> decltype(lhs.get() <= rhs.get())
+                const not_null<U>& rhs) noexcept(noexcept(std::less_equal<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::less_equal<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() <= rhs.get();
+    return std::less_equal<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator>(const not_null<T>& lhs,
-               const not_null<U>& rhs) noexcept(noexcept(lhs.get() > rhs.get()))
-    -> decltype(lhs.get() > rhs.get())
+               const not_null<U>& rhs) noexcept(noexcept(std::greater<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::greater<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() > rhs.get();
+    return std::greater<>{}(lhs.get(), rhs.get());
 }
 
 template <class T, class U>
 auto operator>=(const not_null<T>& lhs,
-                const not_null<U>& rhs) noexcept(noexcept(lhs.get() >= rhs.get()))
-    -> decltype(lhs.get() >= rhs.get())
+                const not_null<U>& rhs) noexcept(noexcept(std::greater_equal<>{}(lhs.get(), rhs.get())))
+    -> decltype(std::greater_equal<>{}(lhs.get(), rhs.get()))
 {
-    return lhs.get() >= rhs.get();
+    return std::greater_equal<>{}(lhs.get(), rhs.get());
 }
 
 // more unwanted operators