Add C++ standards to the build matrix

[jgopel]

Problem:

• A variety of compilers are tested in CI, but they are all tested on
  the C++11 standard. This prevents CI from catching bugs that exist
  only on a particular version of the C++ standard.

Solution:

• Parameterize CI on C++ standards 11, 14, 17, and 20.

This also includes fixes for compiler warnings discovered along the way. I've included a detailed writeup of each compilation error and the fix in the commit notes for each fix.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[jgopel]

I wrote a little script that generates a build directory for every possible compiler version & standard version combination. It's nothing fancy, but getting the details right took a few minutes. Would there be any interest in upstreaming that? Maybe something like support/generate-all-builds.py?

[jgopel]

Looks like this partially duplicates #1992 - not sure how to proceed here.

[alexezeder]

Looks like this partially duplicates #1992 - not sure how to proceed here.

Closed. You may want to read my final words there.

[vitaut]

Thanks for the PR. Looks good in general, just a few minor comments.

[vitaut]

Would there be any interest in upstreaming that? Maybe something like support/generate-all-builds.py?

Probably not but you can post it here in case anyone is interested.

[jgopel]

Would there be any interest in upstreaming that? Maybe something like support/generate-all-builds.py?

Probably not but you can post it here in case anyone is interested.

Here's a gist https://gist.github.com/jgopel/1d9fcdaf1f82c6f0440487cba791ee68

[vitaut]

Closing since the new standards have been added in f8640d4 but thanks for the PR.

[jgopel]

Closing since the new standards have been added in f8640d4 but thanks for the PR.

I'm not sure that does what you want it to do, @vitaut. I had experimented with that approach and it didn't generated both debug and release jobs for those standards. See: https://github.com/jgopel/fmt/actions/runs/361646531 and https://github.com/jgopel/fmt/actions/runs/361658036. The only way I've found success so far is with an approach like this https://github.com/jgopel/fmt/actions/runs/363543436.

Note that there's an open issue with GitHub Actions about roughly this problem. You can find that here: https://github.community/t/reusing-sharing-inheriting-steps-between-jobs-declarations/16851

[jgopel]

I've updated this to reflect my latest thinking about what this feature should look like - including support for using an older OS as discussed by @alexezeder and @darklukee on #1992. No pressure to merge this, just wanted to document what I've learned and put forth my best attempt at a solution that has all the desired attributes.

Edit: It seems that PRs stop updating once they're closed, so at time of writing the SHAs of interest for my final proposed changeset are cddc7ea and cc60505.

[vitaut]

it didn't generated both debug and release jobs for those standards

That's fine.