ENV.cc and ENV.cxx is not correctly set when depending on gcc formula

[iMichka]

What you were trying to do (and why)

Trying to install a meson-based formula (https://github.com/Homebrew/linuxbrew-core/pull/21946) on linux with gcc@7.
The issues is also present on macOS.

What happened (include command output)

ENV["CC"], ENV["CXX"], ENV.cc, ENV.cxx is incorrectly set in the install and test block.

The wrong compiler is defined.

• On Mac it is always set to clang/clang++, even if I define depends_on "gcc@x" in the formula.
• On Linux it is always set to the biggest gcc installed: if gcc@10 is installed on the system, ENV.cc is always gcc-10.

meson (but maybe other build tools) will pick up the wrong compiler.
Some tests blocks are run with the wrong compiler too, as we use ENV.cc a lot.

What you expected to happen

ENV["CC"], ENV["CXX"], ENV.cc, ENV.cxx should point to the gcc formula defined in the formula.

Note

This issue might not be a bug but a design choice. In that case I would like to change that behaviour.

[carlocab]

On Mac it is always set to clang/clang++, even if I define depends_on "gcc@x" in the formula.

I think this is intentional, as many of the formulae just depend on gcc for gfortran. If you want it not to use clang on macOS, I think you need fails_with.

[iMichka]

I see. I can use (on Mac)

  fails_with :clang
  depends_on "gcc@9"

which gives me ... gcc-10, because I have gcc installed.

If I want gcc@9, I need to use:

  fails_with :clang
  fails_with gcc: "10"
  depends_on "gcc@9"

If I want gcc@7, I need to use:

  fails_with :clang
  fails_with gcc: "8"
  fails_with gcc: "9"
  fails_with gcc: "10"
  depends_on "gcc@7"

It's not the shortest syntax but it works.

[carlocab]

Yes. I haven't managed to figure out how to use fails_with to get it to use LLVM clang instead of Apple clang, though. But I didn't try very hard, and that was a while ago.

[iMichka]

Ok so I can live with the fails_with syntax for the time being.

I think the biggest issue I have right now is on Linux: ENV.cc and friends will always point to the highest gcc installed.
Historically brewed gcc on linux was gcc-5. I am changing it to align it with homebrew-core, so now brew gcc becomes gcc-10 (https://github.com/Homebrew/linuxbrew-core/pull/21380).

If I brew install gcc on linux, now all the ENV.cc will point to gcc-10, even with a pristine formula that does not define a specific gcc.

I would expect the following behaviour (on Linux):
ENV.CC points to /usr/bin/gcc by default
ENV.CC points to gcc@5 if /usr/bin/gcc != gcc-5 (this also means that gcc@5 needs to be installed)
ENV.CC points to gcc@x if gcc@x is defined in the formula

I have no clue how to change that behaviour right now.

[jonchang]

The logic is in compilers.rb, but compiler selection is pretty hairy currently and we'd need to think carefully about how to handle this. In particular:

ENV.CC points to gcc@5 if /usr/bin/gcc != gcc-5 (this also means that gcc@5 needs to be installed)

Do we want gcc@5 to be used if the system compiler is newer?

[iMichka]

Do we want gcc@5 to be used if the system compiler is newer?

I would like. Almost everything is compiled with gcc5 on linux CI: I would like to use brewed gcc5 as a fallback when no gcc5 is present. This makes sure the builds have more chances to pass if someone builds from source.

[carlocab]

One thing I noticed though is that fails_with gcc "8" also appears to rule out any newer gcc, so it may be sufficient to just do

fails_with :clang
fails_with gcc: "8"
depends_on "gcc@7"

if you want gcc@7.

[MikeMcQuaid]

Yes, fails_with is what's required to make this work as expected. We can't really change the behaviour of depends_on here without breaking lots of other formulae.

ENV.CC points to /usr/bin/gcc by default
ENV.CC points to gcc@5 if /usr/bin/gcc != gcc-5 (this also means that gcc@5 needs to be installed)

Something like this seems reasonable for Linux-only with a concept of preferring a specific compiler version that we use for building bottles.

[iMichka]

ENV.CC points to /usr/bin/gcc by default
ENV.CC points to gcc@5 if /usr/bin/gcc != gcc-5 (this also means that gcc@5 needs to be installed)

Something like this seems reasonable for Linux-only with a concept of preferring a specific compiler version that we use for building bottles.

This is on the todo list then for the linuxbrew-core migration.

[BrewTestBot]

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