update relocation definitions to track p2786r13 standard by guptapratykshh · Pull Request #6869 · TheHPXProject/hpx

guptapratykshh · 2026-02-02T07:13:49Z

Fixes #6625

Proposed Changes

added is_replaceable trait to align with p2786r13
updated is_trivially_relocatable to use the standard version if the c++26 feature macro is present
wired up relocate_at to call std::relocate_at when available

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Checklist

Not all points below apply to all pull requests.

I have added a new feature and have added tests to go along with it.
I have fixed a bug and have added a regression test.
I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

StellarBot · 2026-02-02T07:15:03Z

Can one of the admins verify this patch?

hkaiser · 2026-02-02T12:52:02Z

@isidorostsa Would you please have a look?

isidorostsa

Hey @guptapratykshh thank you for taking a look at this!

Unfortunately, P2786 was pulled last minute from C++26 in the Kona meeting, so the issue you tackled is outdated, I apologize for that.

That being said, it wouldn't hurt to implement the replaceable trait.

Great code!

isidorostsa · 2026-02-02T16:26:09Z


+#if defined(__cpp_lib_trivially_relocatable)
+    template <typename T>
+    struct is_trivially_relocatable : std::is_trivially_relocatable<T>


this should also be added to the replacabillity definition

this feature test macro should also be updated

isidorostsa · 2026-02-02T16:29:04Z

        // noexcept if the memmove path is taken or if the move path is noexcept
        noexcept(detail::relocate_at_helper(src, dst)))
    {
+#if defined(__cpp_lib_trivially_relocatable)


I don't think std::relocate_at is being proposed, but I'm not sure.

If it is, it would be better to avoid defining all the helpers and replace everything with a using relocate_at.

isidorostsa · 2026-02-02T16:39:31Z

+namespace hpx::experimental {
+
+    HPX_CXX_EXPORT template <typename T>
+    struct is_replaceable


A type is replaceable if what you get from move constructing is the same you get from move assigning.
In code:

T move_assigned = some_t(); T move_constructed(std::move(existing)); // 1 move_assigned = std::move(existing); // 2

You want move_constructed to be the same as move_assigned, and the conditions you've put forth are not enough to understand if this is true.

Replaceable is an opt-in trait, you might want to take a look at how we deal with such traits in the is_trivially_relocatable definition.

Take a look at this paper to learn more about how to detect replaceability.

Are array types not replaceable?

arrays are not replaceable because they are not assignable types. while arrays are objects, they do not support move assignment (std::is_trivially_move_assignable_v<T[]> is false), which is core requirement for replaceability according to P2786R13. the explicit specialisations make this exclusion clear and match the expected behavior shown in the test assertions at lines 35-36.

isidorostsa · 2026-02-02T16:43:35Z

+#include <mutex>
+#include <type_traits>
+
+using hpx::experimental::is_replaceable_v;


Those tests should be updated in accordance with the changes in the implementation of the replacabillity trait.

isidorostsa

Why is this checking for trivial relocatabillity?
How would a user opt in?

I suggest reviewing P2786 to get a tighter grip on the specifics of this type trait.

Thanks!

guptapratykshh · 2026-02-04T16:57:58Z

thanks for the feedback. i have updated is_replaceable to strictly follow p2786r13, remove the trivial relocatability check, and allow users to opt in via specialization.

isidorostsa · 2026-02-07T11:26:25Z

Good work, the trait is mostly correct, now you should add the opt in mechanism and the tests to check for that :)

guptapratykshh · 2026-02-08T16:08:40Z

i have added opt-in mechanism through template specialization, users can now specialize is_replaceable to mark their types as replaceable. i have also added tests showing it works opt_in_replaceable(with specialization) is replaceable while move_assignable_but_not_implicitly_replaceable(without) is not. all tests pass

hkaiser · 2026-02-08T16:41:02Z

@guptapratykshh Could you please rebase your branch onto master (and resolve the conflicts)?

guptapratykshh · 2026-02-10T08:06:24Z

have rebased the branch , please have a look @isidorostsa

guptapratykshh · 2026-02-16T11:47:33Z

ping @isidorostsa @hkaiser

guptapratykshh · 2026-02-22T10:32:47Z

@isidorostsa can you please have a look at this and review , i have addressed all changes that were there

isidorostsa · 2026-02-22T10:34:01Z

@isidorostsa can you please have a look at this and review , i have addressed all changes that were there

Will do, apologies for the delay, there are many pull requests right now

guptapratykshh · 2026-03-03T05:32:08Z

hello @isidorostsa , did you have to time to check and review this

isidorostsa

Appreciate the work, thank you

isidorostsa · 2026-03-03T06:06:58Z

+static_assert(is_replaceable_v<int*>);
+static_assert(is_replaceable_v<int const*>);    // pointer itself is mutable
+static_assert(
+    !is_replaceable_v<int* const>);    // const pointer is not replaceable


Why is that?

const-qualified objects cannot be assigned, so they cannot be replaced

isidorostsa · 2026-03-03T06:08:02Z

+    not_destructible(not_destructible&&);
+    ~not_destructible() = delete;
+};
+static_assert(!is_replaceable_v<not_destructible>);


Is destructibility a requirement of replaceability?

yes, implicit replaceability requires trivial relocatability, which requires trivial destructibility

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

hkaiser · 2026-03-09T12:49:10Z

@guptapratykshh @isidorostsa Any progress here?

isidorostsa · 2026-03-09T12:58:11Z

@guptapratykshh could you please address the questions in the review? Thanks for the work!

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

guptapratykshh · 2026-03-25T07:36:33Z

@isidorostsa can you please review this PR

hkaiser

LGTM, thanks!

guptapratykshh · 2026-03-30T17:11:56Z

hi @isidorostsa, i have commented on the questions that were asked in the previous comments. please have a look at that , have addresed them

guptapratykshh · 2026-04-08T16:19:00Z

hi @isidorostsa, any updates on this PR

codacy-production · 2026-04-12T16:39:22Z

Up to standards ✅

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isidorostsa

@guptapratykshh @hkaiser Sorry for the delay in reviewing this. This just needs some minor fixups and then it will be good to merge.

isidorostsa · 2026-04-19T15:07:50Z

+
+namespace hpx::experimental {
+
+#if defined(__cpp_lib_trivially_relocatable)


The correct feature test macro is __cpp_trivial_relocatability
(https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2786r13.html#language-feature-test-macros)

isidorostsa · 2026-04-19T15:08:21Z


+#if defined(__cpp_lib_trivially_relocatable)
+    template <typename T>
+    struct is_trivially_relocatable : std::is_trivially_relocatable<T>


this feature test macro should also be updated

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Copilot

Pull request overview

This PR updates HPX’s relocatability/type-trait infrastructure to better align with the C++26 direction from P2786R13 by introducing a new is_replaceable trait and preferring standard library relocation facilities/traits when available.

Changes:

Added hpx::experimental::is_replaceable (with tests) to model C++26 “replaceability”.
Updated is_trivially_relocatable to defer to the standard library trait when a feature macro is detected.
Minor adjustments around relocation primitives and relocate_at integration/guarding.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
libs/core/type_support/tests/unit/is_replaceable.cpp	New compile-only unit test coverage for `is_replaceable`.
libs/core/type_support/tests/unit/CMakeLists.txt	Registers `is_replaceable` as a compile-only test.
libs/core/type_support/include/hpx/type_support/uninitialized_relocation_primitives.hpp	Updates `#endif` comment to match the controlling macro.
libs/core/type_support/include/hpx/type_support/relocate_at.hpp	Tweaks include-guarding and refines a static_assert message.
libs/core/type_support/include/hpx/type_support/is_trivially_relocatable.hpp	Uses the standard trait when a C++26 feature macro is detected.
libs/core/type_support/include/hpx/type_support/is_replaceable.hpp	Introduces `is_replaceable` trait with std/fallback implementations.
libs/core/type_support/CMakeLists.txt	Adds the new `is_replaceable.hpp` header to the type_support header set.

Copilot · 2026-04-19T16:17:28Z

+#if defined(__cpp_trivial_relocatability)
+    HPX_CXX_CORE_EXPORT template <typename T>
+    struct is_replaceable : std::is_replaceable<T>
+    {
+    };


This branch assumes std::is_replaceable<T> exists whenever __cpp_trivial_relocatability is defined, but __cpp_trivial_relocatability is a core-language macro and does not guarantee that the standard library provides std::is_replaceable in <type_traits>. This can lead to hard compile failures on toolchains where the macro is defined but the library trait is missing (or to missed use of the standard trait when only the library macro is defined). Prefer checking the library feature-test macro (or an HPX HPX_HAVE_* configure-time check) that specifically indicates std::is_replaceable support.

What do you think about this comment? Would you mind explaining why you resolved it?

These are the comments: #6869 (comment) & #6869 (comment) that prompted commit 657e7f7 ("fix feature test macro to use __cpp_trivial_relocatability per p2786r13"). isidorostsa explicitly named the macro and linked to the exact P2786R13 section. So when Copilot later suggested __cpp_lib_trivially_relocatable, that suggestion directly contradicted (and pointed at a macro the paper does not define)

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

hkaiser

@guptapratykshh Do you plan to address @isidorostsa's comments?

Copilot

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

guptapratykshh requested a review from hkaiser as a code owner February 2, 2026 07:13

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from 6d446e2 to 2688475 Compare February 2, 2026 07:21

hkaiser added type: enhancement type: compatibility issue category: algorithms labels Feb 2, 2026

isidorostsa suggested changes Feb 2, 2026

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guptapratykshh requested a review from isidorostsa February 3, 2026 10:51

isidorostsa suggested changes Feb 3, 2026

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guptapratykshh requested a review from isidorostsa February 4, 2026 16:58

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from c119545 to 254bc27 Compare February 8, 2026 18:30

isidorostsa suggested changes Mar 3, 2026

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guptapratykshh added 3 commits March 3, 2026 12:29

update relocation semantics to match p2786r13

3840ea4

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

relocation traits

a5585d2

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

align is_replaceable with p2786 semantics and allow user opt-in

df80f76

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from 1f820c0 to 976a1da Compare March 3, 2026 18:28

address maintainer feedback on is_replaceable semantics and tests

37063e4

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from 976a1da to 37063e4 Compare March 3, 2026 19:39

addresed feedback on is_replaceable semantics and test coverage

9d857b6

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from 8a482fc to 9d857b6 Compare March 16, 2026 14:04

guptapratykshh requested a review from isidorostsa March 20, 2026 06:48

hkaiser previously approved these changes Mar 26, 2026

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isidorostsa suggested changes Apr 19, 2026

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fix feature test macro to use __cpp_trivial_relocatability per p2786r13

657e7f7

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Copilot AI review requested due to automatic review settings April 19, 2026 16:11

guptapratykshh dismissed hkaiser’s stale review via 657e7f7 April 19, 2026 16:11

Copilot started reviewing on behalf of guptapratykshh April 19, 2026 16:11 View session

Copilot AI reviewed Apr 19, 2026

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remove redundant include guard and clarify macro usage

2e6decb

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Copilot AI review requested due to automatic review settings April 20, 2026 05:10

guptapratykshh force-pushed the fix/issue-6625-update-relocation branch from 7a6cd7b to 2e6decb Compare April 20, 2026 05:10

Copilot started reviewing on behalf of guptapratykshh April 20, 2026 05:10 View session

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fix includes, macro guards, and test header

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guptapratykshh requested a review from isidorostsa April 21, 2026 09:16

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restore __cpp_trivial_relocatability per p2786r13 reviewer feedback

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		namespace hpx::experimental {

		#if defined(__cpp_lib_trivially_relocatable)

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guptapratykshh commented Feb 2, 2026

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