CI GHC 9.4-alpha and edit Cabal/src/Distribution/Simple/GHC.hs to accept GHC 9.4

[Mikolaj]

Apparently cabal now yells

"Warning: Unknown/unsupported 'ghc' version detected (Cabal 3.8.0.20220526 supports 'ghc' version < 9.4)"?

when somebody uses GHC 9.4 and that's rather embarrassing and puts a pressure on us to create another point release ASAP when GHC 9.4-final is released. It would be ideal to add GHC 9.4 to CI and to Cabal/src/Distribution/Simple/GHC.hs (I can do the latter rather easiliy) before cabal's 3.8 release.

[Mikolaj]

This is blocked on haskell/actions#93 and haskell/actions#107.

In the worst case, we are going to release 3.8 without support for GHC 9.4 and, at some point, we may push a point release to support GHC 9.4. If haskell/actions doesn't support GHC 9.4, that's a good enough reason to yell "Unknown/unsupported 'ghc' version detected".

[andreasabel]

Haskell-CI supports 9.4: haskell-CI/haskell-ci#588

[fgaz]

Or we can add it without ci. A local test run should be enough, there isn't much to brwak

[Mikolaj]

Yes, it's our own decision

1. how much weight should our formal recognition of a GHC version carry
2. how alarming the "unsupported" warning should be
3. how spammy we want to be when the user uses an unsupported GHC version

My gut feeling is

1. once we recognize a GHC version, we need a single full test/bootstrap/etc run on major architectures for each new point release of the major release that started supporting the new GHC; normally CI does that for us, but this can be done manually every time on gitlab, I suppose; volunteers welcome
2. the warning is just right
3. we could be less spammy, e.g., record the offending GHC version somewhere in dist-newstyle and don't warn any more about that GHC until cabal clean

[jneira]

Haskell-CI supports 9.4: haskell-CI/haskell-ci#588

Nice!, if haskell-ci is able to generate a github workflow which uses ghc alpha/pre releases via templates, we can do the same writing a direct workflow at hand and using our validate.sh script.
@robx tried to do it with gha and then tried to replace gha with ghcup.
But maybe we could do a separate workflow if it is too different to existing ones.

[gbaz]

I ran a regression testsuite (cabal-testsuite) against preleaseses of cabal and ghc on macos both x86 and M1.

cabal-tests --with-cabal /Users/myuser/.ghcup/bin/cabal --with-ghc /Users/myuser/.ghcup/ghc/9.4.0.20220623/bin/ghc

There were a ton of unexpected fails, but eyeball inspection reveals, best I can tell, that this is because the golden output differed precisely because of the unexpected addition of the above warning (namely: "Warning: Unknown/unsupported 'ghc' version detected (Cabal 3.8.0.20220526 supports 'ghc' version < 9.4)").

To reiterate our verbal discussion -- the purpose of releasing cabal 3.8 is to support 9.4, and releasing it with this warning would be very silly, and we should fix it. This is to say that 9.4 is in our support window -- i.e. cabal 3.8 not working with 9.4 would be a bug, while cabal 3.6 not working with 9.4 would simply be "an unfortunate consequence that can be resolved by updating to the latest cabal"

[gbaz]

Oh -- further motivation for changing this -- the warning is not in cabal-install, but Cabal-the-library. So its double-nuts to have the library that literally ships with ghc 9.4 warn about not supporting it!