Fix compilation with re2 2023-07-01

[stanhu]

https://code-review.googlesource.com/c/re2/+/61250 made re2 depend directly on abseil. The latest version of abseil now requires C++14 (https://github.com/abseil/abseil-cpp/issues/1127). However, it seems that supplying -std=c++14 isn't enough (https://github.com/abseil/abseil-cpp/issues/1431). -std=c++17 needs to be used at least.

This commit fixes the compilation by trying C++20 and C++17 if the initial compilation fails. This fixes build issues on macOS.

[stanhu]

It seems abseil is no longer an optional dependency on a separate branch: https://github.com/google/re2/issues/388

https://code-review.googlesource.com/c/re2/+/61270 updated the README to make this official.

[mudge]

Thanks for this, @stanhu. I'll try and get the new 2023-07-01 release (with its added abseil dependency) into the CI matrix (via re2-ci).

[mudge]

Have you had any luck compiling the latest re2 from scratch on Ubuntu 20.04?

[stanhu]

Have you had any luck compiling the latest re2 from scratch on Ubuntu 20.04?

@mudge Yes, but only by installing a PPA with libabsl-dev. It seems that the libabsl-dev package is only available with Ubuntu 22.04: https://packages.ubuntu.com/jammy/libabsl-dev. This is what I ran:

docker run -it ubuntu:20.04 bash
apt update && apt install -y software-properties-common build-essential pkg-config git
add-apt-repository ppa:savoury1/build-tools
apt install -y libabsl-dev
git clone https://github.com/google/re2.git /tmp/re2
cd /tmp/re2
make

[mudge]

I hoped I could bump the more recent Ruby builds to Ubuntu 22.04 so we could use libabsl-dev (and libabsl20210324) but it looks like there are fatal problems trying to use RubyGems on Ubuntu 22.04 so I'll need to use the PPA or package up my own libabsl and stick with Ubuntu 20.04 for older Rubies.

[stanhu]

@mudge Is there a reason these older Ruby versions still need to be supported? Even Ruby 2.7 reached end-of-life 3 months ago.

[mudge]

It's not a popular opinion but until it becomes extraordinarily painful (which might not be too far away), I don't want to break compatibility for anyone who can already run the gem.

In this case, I've added the PPA you mentioned to the existing 20.04 build (and I've packaged up a .deb of re2 2023-07-01 compiled against the libabsl-dev from the PPA) but it looks like the build is failing: https://github.com/mudge/re2/actions/runs/5456057642/jobs/9928355646

[mudge]

OK, I've fixed the build (libabsl2206 isn't enough, we needed the full libabsl-dev).

[mudge]

Thanks @stanhu, I've now rebased this into https://github.com/mudge/re2/commit/8b62c5c4ac5552d6b3465d0d4a3b6494604bcb1b and it'll go out in version 1.7.0 once the tests pass on main.