[KR-Ravindra]

Description

Fixes #2654

Problem

Since C++17, noexcept is part of the function type. function_traits has no specializations for noexcept callables, so they fall through to the primary template, which then fails on &FunctionT::operator(). Callbacks like a noexcept free function, [](int) noexcept {}, or std::bind(&C::cb, this, _1) with void cb(int) noexcept don't compile when passed to APIs that use function_traits.

Fix

This adds noexcept specializations that forward to the existing ones. They cover free functions, function pointers, const call operators (lambdas), and std::bind of noexcept methods (const and non-const) and free functions, in every standard-library branch the file already handles. Callbacks are still not required to be noexcept, per the discussion in the issue.

How tested

There's a new TestFunctionTraits.noexcept_callables gtest in test_function_traits.cpp. I couldn't run colcon test locally, so I compiled and ran test_function_traits.cpp on its own against the header (it only needs the standard library):

• Apple clang 21 / libc++ (macOS): 8/8 pass. With the current header, it fails to compile on every noexcept case.
• GCC 14 / libstdc++ (gcc:14 container), -Wall -Wextra -Werror: 8/8 pass, against 25 compile errors with the current header.
• The MSVC branch mirrors the existing one but is untested locally, so please rely on the Windows CI for it.
• cpplint with the ament line length is clean. I haven't run ament_uncrustify.

Is this user-facing behavior change?

Yes: noexcept callables can now be used as callbacks.

Did you use Generative AI?

Yes. The change, tests and this description were prepared with Claude Code (Claude Opus 5.5), an AI agent operated by KR-Ravindra, who reviewed and tested it. The agent is pointed only at issues maintainers have confirmed and problems he has hit, not at producing contributions.

[github-actions]

ABI Compliance Check

✅ Verdict: compatible

Library	Verdict	Summary
libcomponent_manager.so	✅ compatible	No ABI changes detected.
librclcpp.so	✅ compatible	No ABI changes detected.
librclcpp_action.so	✅ compatible	No ABI changes detected.
librclcpp_lifecycle.so	✅ compatible	No ABI changes detected.

✅ libcomponent_manager.so — full abidiff report

Compared:

• Base: lib-base/libcomponent_manager.so
• Head: lib-pr/libcomponent_manager.so @ 9485c0f

(empty report — no differences printed by abidiff)

✅ librclcpp.so — full abidiff report

Compared:

• Base: lib-base/librclcpp.so
• Head: lib-pr/librclcpp.so @ 9485c0f

(empty report — no differences printed by abidiff)

✅ librclcpp_action.so — full abidiff report

Compared:

• Base: lib-base/librclcpp_action.so
• Head: lib-pr/librclcpp_action.so @ 9485c0f

(empty report — no differences printed by abidiff)

✅ librclcpp_lifecycle.so — full abidiff report

Compared:

• Base: lib-base/librclcpp_lifecycle.so
• Head: lib-pr/librclcpp_lifecycle.so @ 9485c0f

(empty report — no differences printed by abidiff)

_{Updated for commit 9485c0f · suppressions: /home/runner/work/_temp/ros2-abi-suppressions.txt}

[fujitatomoya]

lgtm with green CI.

[fujitatomoya]

Pulls: #3322
Gist: https://github.com/proxy/gist.githubusercontent.com/fujitatomoya/00c2df441c1c534bc665658a5962ab38/raw/6731d30fd44b870565de68785845495c98400b83/ros2.repos
BUILD args: --packages-above-and-dependencies rclcpp
TEST args: --packages-above rclcpp
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/20753

• Linux
• Linux-aarch64
• Linux-rhel
• Windows