Update Linux GCC code.

[MikeMcQuaid]

Update both the variables that dictate this and the documents that explain our GCC/glibc policies.

These should ease a later future migration to a newer GCC version.

This is based on the conversation in #19238.

[Bo98]

Need to install g++-12 in the Docker image (on 22.04 only) and will require adjusting the needs_compiler_formula? check to check runtime libraries rather than the compiler. Ubuntu 22.04 is a weird one where the default compiler /usr/bin/gcc is 11 but the runtime libraries (which is what really matters 99% of the time) is 12.

Might also require compiler selector changes if it is preferring /usr/bin/gcc over /usr/bin/gcc-12. The compiler selector should already support the latter - just a matter of whether it prioritises it correctly or not. Just noting it to make sure it is tested.

[MikeMcQuaid]

Thanks @Bo98. @iMichka is this something you (or another @Homebrew/maintainers who meaningfully uses Linux, I don't really) could finish off?

[iMichka]

Ok so after thinking a little bit about this

• I'm fine with bumping the compiler version but being conservative about glibc.
• Need to install g++-12 in the Docker image -> can we uninstall gcc 11 to gain some space and not bloat the image (I do not know the impact on image size)
• How will users know they need to switch to gcc12 if possible?
• I would like to see is a deadline for switching to gcc 13: if we follow LTS versions this will not happen before 2027 and I think this is too far away from now?

[MikeMcQuaid]

• Need to install g++-12 in the Docker image -> can we uninstall gcc 11 to gain some space and not bloat the image (I do not know the impact on image size)

@iMichka I don't know, I'm OK with trying!

• How will users know they need to switch to gcc12 if possible?

Feels like we should just handle this automatically for them with Docker images/formulae etc.

Otherwise: perhaps a brew doctor suggestion or adding to the installer?

• I would like to see is a deadline for switching to gcc 13: if we follow LTS versions this will not happen before 2027 and I think this is too far away from now?

What's the need to move to GCC 13? What is blocked by being on GCC 12 (or even 11)?

[MikeMcQuaid]

I've hit my time box of what I can justify spending on this for now.

I've removed the actual version bumps here and scoped this to the refactoring/fixes that were needed that work without the version bumps.

The tests (bar formula audit failing which I don't understand) were passing here so this is ready to merge as-is and will make life easier for someone wanting to update these variables in future if they look back at the changes in 98b9ab0 to Dockerfile, the new host_gcc_path in Library/Homebrew/extend/os/linux/development_tools.rb and bumping versions in Library/Homebrew/os.rb. The remaining points are those raised by @iMichka, fixing the formula audit job and doing testing on a Linux machine.

[p-linnane]

Thanks!