Add constexpr to not_null comparison operators (#1208)

* Initial plan for issue

* Add test and plan to make not_null comparison functions constexpr

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

* Add constexpr to not_null comparison operators

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

* Fix copyright year in constexpr_notnull_tests.cpp

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

* Fix constexpr tests for better compiler compatibility

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

* Remove build artifacts and update .gitignore

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

* Fix constexpr tests to be compatible with more compilers

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

* copilot: Provide more project context for the Copilot coding agent (#1207)

* copilot: create .github/copilot-instructions.md

This file provides additional context and instructions to GitHub
Copilot so it can better understand the codebase and coding conventions.

More can be found about this file at the following links:
 - [Best practices for using Copilot to work on tasks](https://docs.github.com/en/enterprise-cloud@latest/copilot/using-github-copilot/using-copilot-coding-agent-to-work-on-tasks/best-practices-for-using-copilot-to-work-on-taskshttps://docs.github.com/en/enterprise-cloud@latest/copilot/using-github-copilot/using-copilot-coding-agent-to-work-on-tasks/best-practices-for-using-copilot-to-work-on-tasks)
 - [Adding repository custom instructions for GitHub Copilot](https://docs.github.com/en/enterprise-cloud@latest/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot?tool=webuihttps://docs.github.com/en/enterprise-cloud@latest/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot)

* copilot: add copilot-setup-steps.yml

This new workflow is done when copilot loads into an environment and
enables copilot to be sure it has the proper dependencies before working
on changes. Also included in the change are explicit instructions on
what to do before reporting back "done".

* Initial plan for issue

* Rebase onto main and verify changes meet project guidelines

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

* Update .gitignore to exclude build-cxx* directories

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

* Fix newline at end of constexpr_notnull_tests.cpp and update .gitignore

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

* Remove C++14 feature check that is redundant since C++14 is the minimum supported standard

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

---------

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

Author: Copilot

.gitignore

@@ -1,5 +1,5 @@
 CMakeFiles
-build
+build*/
 tests/CMakeFiles
 tests/Debug
 *.opensdf

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

tests/constexpr_notnull_tests.cpp

@@ -0,0 +1,74 @@
+///////////////////////////////////////////////////////////////////////////////
+//
+// Copyright (c) 2025 Microsoft Corporation. All rights reserved.
+//
+// This code is licensed under the MIT License (MIT).
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include <gsl/pointers> // for not_null
+#include <gtest/gtest.h>
+
+#include <type_traits> // for declval
+
+using namespace gsl;
+
+namespace
+{
+constexpr bool comparison_test(const int* ptr1, const int* ptr2)
+{
+    const not_null<const int*> p1(ptr1);
+    const not_null<const int*> p1_same(ptr1);
+    const not_null<const int*> p2(ptr2);
+    
+    // Testing operator==
+    const bool eq_result = (p1 == p1_same); // Should be true
+    const bool neq_result = (p1 != p2);     // Should be true
+    
+    // Testing operator<= and operator>=
+    const bool le_result = (p1 <= p1_same); // Should be true
+    const bool ge_result = (p1 >= p1_same); // Should be true
+    
+    // The exact comparison results will depend on pointer ordering,
+    // but we can verify that the basic equality checks work as expected
+    return eq_result && neq_result && le_result && ge_result;
+}
+
+constexpr bool workaround_test(const int* ptr1, const int* ptr2)
+{
+    const not_null<const int*> p1(ptr1);
+    const not_null<const int*> p1_same(ptr1);
+    const not_null<const int*> p2(ptr2);
+    
+    // Using .get() to compare
+    const bool eq_result = (p1.get() == p1_same.get()); // Should be true
+    const bool neq_result = (p1.get() != p2.get());     // Should be true
+    
+    return eq_result && neq_result;
+}
+} // namespace
+
+constexpr int test_value1 = 1;
+constexpr int test_value2 = 2;
+
+static_assert(comparison_test(&test_value1, &test_value2), "not_null comparison operators should be constexpr");
+static_assert(workaround_test(&test_value1, &test_value2), "not_null .get() comparison workaround should work");
+
+TEST(notnull_constexpr_tests, TestNotNullConstexprComparison)
+{
+    // This test simply verifies that the constexpr functions compile and run
+    // If we got here, it means the constexpr comparison operators are working
+    static const int value1 = 1;
+    static const int value2 = 2;
+    EXPECT_TRUE(comparison_test(&value1, &value2));
+    EXPECT_TRUE(workaround_test(&value1, &value2));
+}
+