"panic! (the 'impossible' happened)" on 9.4 series

[tomjaguarpaw]

I'm getting panic! (the 'impossible' happened) on 9.4 series compilers when compiling Cabal-syntax-3.14.0.0. For example, stan just failed on all of

      - "9.4.1"
      - "9.4.2"
      - "9.4.4"
      - "9.4.5"
      - "9.4.6"

[EDIT: remove 9.4.7, because it actually works]

(stan doesn't support 9.4.3, and I haven't added 9.4.8 yet) and extensions failed on 9.4.2.

9.4.8 seems to be fine (see https://github.com/kowainik/extensions/actions/runs/11085510928/job/30802009359).

Ultimately this is a GHC bug, but I'm reporting here for visibility.

[ffaf1]

It most likely is !23762.

Note that your CI on 9.4.7 is green, not red.

I wonder:

• Why does our CI not break?
• If there is anything else we can do bar closing this with not our bug.

[geekosaur]

Sounded to me like he just wants it documented somewhere?

[geekosaur]

More to the point, I would document that GHC HQ only supports (if at all) the latest minor version of each major version, and give this and maybe 9.0.1 (severe codegen and gc bugs) as examples of specific versions to be avoided.

Also, our CI is fine because we only test 9.4.8, which he reported as working.

[tomjaguarpaw]

If there is anything else we can do bar closing this with not our bug.

Really I just wanted this recorded somewhere so that when other people hit the issue, and they search for the error message, they will find something explaining what they should do. I guess the answer is:

"If you're using the 9.4 series, only use 9.4.7 or 9.4.8"

[ffaf1]

I see, it makes sense.

Let's add lookupIdSubst to the discussion, because this is the specific GHC function that panics (included in the error).