GH-48973: [R][C++] Fix RE2 compilation errors under C++20

[jonkeane]

Rationale for this change

Fix a building RE2 with C++20

What changes are included in this PR?

The fix, a test

Are these changes tested?

Yes

Are there any user-facing changes?

No

• GitHub Issue: [R][C++] Fix RE2 compilation errors under C++20 #48973

[jonkeane]

@github-actions crossbow submit test-r-fedora-clang

[github-actions]

Revision: 16cd49b

Submitted crossbow builds: ursacomputing/crossbow @ actions-d22016819b

Task	Status
test-r-fedora-clang

[jonkeane]

@github-actions crossbow submit test-r-fedora-clang

[github-actions]

Revision: 654937b

Submitted crossbow builds: ursacomputing/crossbow @ actions-6b659d9faf

Task	Status
test-r-fedora-clang

[jonkeane]

This resolves the warning in this environment (and we now have a test for Fedora+clang with libc++. AFAICT there's no current pre-built fedora clang R image these days.)

The warning seems harmless, anonymous types declared in an anonymous union are an extension but curious if anyone thinks this is something we should upstream a change to re2 for (cc @kou @pitrou)

[kou]

+1

[kou]

We don't need this check.

[jonkeane]

Ah, yes of course it already targets re2. I will fix that. Thanks

[jonkeane]

The failure is on main + being worked on separately: #48961

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 64ce4bd.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 4 possible false positives for unstable benchmarks that are known to sometimes produce them.