brew test CompilerSelectionError when using fails_with & build-only compiler dependency

[cho-m]

brew config output

==> Running Setup#run!
==> brew install-bundler-gems
==> brew config
HOMEBREW_VERSION: 3.2.5-17-g689c1fd
ORIGIN: https://github.com/Homebrew/brew
HEAD: 689c1fdb6ec2637964a004727a4f407861809bbd
Last commit: 6 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: bdbfc4fcbcdef82b3d06c8ecb85c2464c61512bb
Core tap last commit: 6 minutes ago
Core tap branch: master
HOMEBREW_PREFIX: /home/linuxbrew/.linuxbrew
HOMEBREW_BOOTSNAP: set
HOMEBREW_CASK_OPTS: []
HOMEBREW_CLEANUP_PERIODIC_FULL_DAYS: 3650
HOMEBREW_COLOR: set
HOMEBREW_DEVELOPER: set
HOMEBREW_FAIL_LOG_LINES: 150
HOMEBREW_FORCE_HOMEBREW_ON_LINUX: set
HOMEBREW_GITHUB_API_TOKEN: set
HOMEBREW_GIT_EMAIL: 1589480+BrewTestBot@users.noreply.github.com
HOMEBREW_GIT_NAME: BrewTestBot
HOMEBREW_MAKE_JOBS: 2
HOMEBREW_NO_AUTO_UPDATE: set
HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: set
HOMEBREW_NO_EMOJI: set
Homebrew Ruby: 2.6.3 => /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/2.6.3_2/bin/ruby
CPU: dual-core 64-bit skylake
Clang: N/A
Git: 2.32.0 => /usr/bin/git
Curl: 7.47.0 => /usr/bin/curl
Kernel: Linux 5.8.0-1039-azure x86_64 GNU/Linux
OS: Ubuntu 16.04.7 LTS (xenial)
Host glibc: 2.23
/usr/bin/gcc: 5.4.0
/usr/bin/ruby: N/A
glibc: N/A
gcc@5: N/A
xorg: N/A

brew doctor output

==> brew doctor
All steps passed!

• I ran brew update and am still able to reproduce my issue.
• I have resolved all warnings from brew doctor and that did not fix my problem.

What were you trying to do (and why)?

Linux CI run on envoy Homebrew/homebrew-core#82077 with diff to disable default GCC for build (fails_with gcc: "5") and only enable working GCC in necessary section, i.e. build (depends_on "gcc@9" => :build).

EDIT: Issue is more for LLVM rather than GCC since GCC-dependency often carries newer runtime library dependency. Specific examples:

• dotnet dotnet 5.0.205 homebrew-core#81945 when trying to only enable llvm for build.
• swift
• fibjs

It would also be more accurate to test without extra brewed gcc/llvm dependency to make sure it truly isn't used by runtime.

What happened (include all command output)?

brew test fails during set up due to compiler selection error. The error message complains about being unable to "build" the formula, even though it should be testing an already built/installed formula.

See https://github.com/Homebrew/homebrew-core/runs/3190458966

==> FAILED
Error: test failed
==> Testing envoy
Error: envoy: failed
An exception occurred within a child process:
  CompilerSelectionError: envoy cannot be built with any available compilers.
Install Clang or run `brew install gcc`.

What did you expect to happen?

brew test should not fail on CompilerSelectionError. It should run in the test block as is, without requiring extra/unneeded compiler dependencies.

Some clarification/changes to fails_with is necessary so that there is a way to just disable GCC-5(Linux) or Apple Clang in build, but allow usage during test.

Step-by-step reproduction instructions (by running brew commands)

https://github.com/Homebrew/homebrew-core/runs/3190458966

[MikeMcQuaid]

Both of these don't need the compiler for runtime/test.

They don't need for runtime but they do for testing because that's how Homebrew works. This is something I'd review a PR for but I don't see as being a high priority given the workaround is so trivial on our side.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[carlocab]

Related issue, with no workarounds that I know of: Homebrew/homebrew-core#86826 (comment) See @Bo98's comments for an explanation of why it's related.