Comma expressions rejected in #if for C++23

[kyuupichan]

#if 1 ? 1, 0: 3
#error
#endif

clang -E --std=c++23 --pedantic-errors /tmp/bug.cpp gives:

tmp/bug.cpp:1:10: error: comma operator in operand of #if [-Werror,-Wpedantic]
    1 | #if 1 ? 1, 0: 3
      |         ~^ ~


1 error generated.

I believe that the comma operator has been permitted in constant expressions in C++ since about C++11. If true, this should be accepted.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Neil Booth (kyuupichan)

#if 1 ? 1, 0: 3 #error #endif

clang -E --std=c++23 --pedantic-errors /tmp/bug.cpp gives:

tmp/bug.cpp:1:10: error: comma operator in operand of #if [-Werror,-Wpedantic]
    1 | #if 1 ? 1, 0: 3
      |         ~^ ~

1 error generated.

I believe that the comma operator has been permitted in constant expressions in C++ since about C++11. If true, this should be accepted.

[shafik]

Implementations seems to agree on this one: https://godbolt.org/z/P14hqzETa

[shafik]

This appears to be cwg1436

[pinskia]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113571 is the corresponding GCC issue about this.

[cor3ntin]

I think we just need to write tests for this issue.

[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: Neil Booth (kyuupichan)

```cpp #if 1 ? 1, 0: 3 #error #endif ```

clang -E --std=c++23 --pedantic-errors /tmp/bug.cpp gives:

tmp/bug.cpp:1:10: error: comma operator in operand of #if [-Werror,-Wpedantic]
    1 | #if 1 ? 1, 0: 3
      |         ~^ ~


1 error generated.

I believe that the comma operator has been permitted in constant expressions in C++ since about C++11. If true, this should be accepted.

[swote-git]

hello! @cor3ntin Can you assign this issue to me?

[swote-git]

@cor3ntin @tbaederr , I make a PR for this issue.

If you have some spare time, could you please review my PR?