I forgot to write that this is done. Thanks Jendrik for the quick reviews!

The changes look good to me, thanks!

I extended the scope of this issue to an overall update of the build 
environment, following https://www.fast-downward.org/ForDevelopers/Requirements.

- use clang-tidy-12
- changes in workflow checks:
  - MacOS10.15 -> MacOS11 (corresponds to macos-latest)
  - drop Ubuntu 18.04, include 22.04 (with python10, gcc11/12, clang14)
  - Python 8 in Windows/Mac tests and Ubuntu builds

Clang14 complained about unused but set variables that we indeed only use in 
assertions. I protected these variables with preprocessor directives.

I created a pull request: https://github.com/aibasel/downward/pull/125

Please not that I did not yet include the change suggested by Jendrik in 
issue865 but reopened the discussion there.

Our tests currently hardcode clang-tidy-8, which is not available in the default Ubuntu 22.04 repositories. We discussed on Discord how to address this, in particular whether we want to continue to hardcode a specific clang-tidy version. The answer was "yes". We want to move to clang-tidy-12, which is the only version available in the intersection of the default repositories of both Ubuntu 20.04 and Ubuntu 22.04.

It looks like this could be a simple change involving only an update of the version number in a total of 4 places in misc/style/run-clang-tidy.py and .github/workflows/style.yml, but perhaps tests will uncover difficulties. Any volunteers to look into this?

Since we're now well past clang-tidy 6.0, we can also now do what Jendrik suggested in issue856. See that issue and the comment that mentions it in misc/style/run-clang-tidy.py. I think we can do this change in this issue as well, but then we should update the title to something like "move to newer clang-tidy version and enable some additional checks".