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not_null comparison functions are missing constexpr #1203
Description
Describe the bug
The comparison operators for not_null such as operator== and operator!= are not declared constexpr, however not_null can be used in constexpr by using .get() instead, so this seems like an oversight.
To Reproduce
#include <gsl/pointers>
constexpr bool example_error(gsl::not_null<int*> const a, gsl::not_null<int*> const b) noexcept
{
return a == b; //error
}
constexpr bool example_workaround(gsl::not_null<int*> const a, gsl::not_null<int*> const b) noexcept
{
return a.get() == b.get(); //works
}Expected behavior
The comparison operators should be constexpr so that the workaround is not necessary.
Spec (please complete the following information):
- OS: Windows
- Compiler: both MSVC & ClangCL (VS 2022, 17.13.6)
- C++ Version: C++17
Additional context
None
Reacted by Copilot
Activity
carsonRadtke commentedon Apr 30, 2025
Thanks for reporting this issue!
I suspect this is not an oversight, your workaround requires
gsl::not_null<T>::element_typeto have a constexproperator==. Concretely, https://godbolt.org/z/zrqze4sbj will not work with the provided workaround.LB-- commentedon May 1, 2025
From my understanding of the C++ standard, a template function can be marked
constexpras long as at least one template instantiation allows it to be used in constexpr evaluation. When it is instantiated in a way that it can't be used in constant evaluation, it just silently ignores theconstexprspecifier. This avoids any need to have a conditionalconstexprsystem in the language, letting the compiler make the determination. Therefore, marking those operators asconstexprshould not break any existing code.carsonRadtke commentedon May 2, 2025
Ah! You're right, thanks for teaching me :)
For completeness, it appears the relevant part of the standard is dcl.constexpr#3.2:
I will raise this issue at the next maintainer's sync, and we will discuss how to proceed.
carsonRadtke commentedon May 21, 2025
Maintainer's call: Looks like an oversight, update the signature to be
constexpr