[clang] set feature test macro ` __cpp_deduction_guides` for P1814R0

[h-vetinari]

The paper (#54051) was mostly implemented in #77890 & #89358, where @cor3ntin said:

[...] I suspect that we do want to finalize the feature for Clang 19.

I cannot tell from browsing the various PRs/issues what (if anything) is still left to do, so opening this issue to track (AFAICT, there are some follow-up issues (e.g. #92224), but they don't look like they have any user-visible impact).

CC @hokein

[h-vetinari]

#55876 might also be closable in the context of this effort (or already now based on the implementation in Clang 19).

[hokein]

Yeah, I don't see any remaining big issues for this feature now. We should definitely set this feature flag in clang 20.
But before that, I'd like to get #103016 fixed.

#55876 might also be closable in the context of this effort (or already now based on the implementation in Clang 19).

This doesn't seem to be related to this feature.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (h-vetinari)

The paper (#54051) was mostly implemented in https://github.com//pull/77890 & #89358, where @cor3ntin [said](https://github.com//pull/89358#discussion_r1575876411): > [...] I suspect that we do want to finalize the feature for Clang 19.

I cannot tell from browsing the various PRs/issues what (if anything) is still left to do, so opening this issue to track (AFAICT, there are some follow-up issues (e.g. #92224), but they don't look like they have any user-visible impact).

CC @hokein

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: None (h-vetinari)

The paper (#54051) was mostly implemented in https://github.com//pull/77890 & #89358, where @cor3ntin [said](https://github.com//pull/89358#discussion_r1575876411): > [...] I suspect that we do want to finalize the feature for Clang 19.

I cannot tell from browsing the various PRs/issues what (if anything) is still left to do, so opening this issue to track (AFAICT, there are some follow-up issues (e.g. #92224), but they don't look like they have any user-visible impact).

CC @hokein

[cor3ntin]

@hokein Is this still on your radar?

[hokein]

Yeah, it is on my radar. My plan is to wait for #98788 to be landed, and then fix #103016 (which needs some code from #98788).