compilers: drop support for GCC 4.9, 5 and 6

[cho-m]

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GCC 4.9 was removed.

GCC 5-8 are all disabled:

• 5 - removal on 2024-10-03
• 6 - removal on 2024-10-03
• 7 - removal on 2025-02-22
• 8 - removal on 2025-02-22

More in line with https://docs.brew.sh/Manpage#install-options-formulacask-

This option will only accept compilers that are provided by Homebrew or bundled with macOS.

[Bo98]

LGTM but might need to wait for the next minor release as this is technically a breaking change?

[MikeMcQuaid]

I don't feel strongly either way. I'd be amazed if there's people relying on GCC 4.9 support given it's been removed for a while.

[Bo98]

System GCC also counts here but yeah.

If we're planning to also drop 5 and 6 maybe it's worth doing it all at the same time which is probably roughly when we'd do a minor release anyway.

[cho-m]

System GCC also counts here but yeah.

Did we support using old system GCC?

I guess this could impact someone still running something like Ubuntu 14.04 LTS or RHEL7/DTS3. That would probably be an awful experience as various formulae have moved to C++17 (e.g. working on ICU 75 PR which makes C++17 a minimum deep in the dependency tree).

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If we're planning to also drop 5 and 6 maybe it's worth doing it all at the same time which is probably roughly when we'd do a minor release anyway.

Makes sense. I was thinking of how many to drop but went conservative with only 4.9.

Minimum GCC 7 may allow cleaning up some fails_with gcc: "5" we added back when running older Ubuntu in CI as GCC 7 has most of the C++17 features.

[Bo98]

Did we support using old system GCC?

For Homebrew/core, not really. Can override it with --cc but that's like maximum levels of untested.

Third party taps are however free to use their own policies and CI images. We recommend aligning with Homebrew/core but do not require it.

[cho-m]

I'll split out documentation changes to separate PR so this only touches Library/Homebrew/compilers.rb

EDIT: #18228

[Bo98]

For when we merge this, it might be worth dropping the gcc-4.9, gcc-5 and gcc-6 shim symlinks too.

[cho-m]

For when we merge this, it might be worth dropping the gcc-4.9, gcc-5 and gcc-6 shim symlinks too.

Forgot about those. Will update PR to remove them.

Also, unrelated to this PR, was looking at shims and noticed we still have some old Apple GCC 4.2 shims. Is there any use for these?

[Bo98]

Minimum Xcode we support is 7.3 so probably can be removed.

I guess technically speaking a formula or build-script could rely on them my invoking llvm-gcc, so the shims do still have some purpose in "modernising" old build scripts to point to newer compilers. This is probably rare but given we're already calling the GCC removal a breaking change, we can bundle removing that with this PR.

[MikeMcQuaid]

As next release is Homebrew 4.4.0: we can remove this now. Thanks @cho-m!