GitLab

I'm synthesizing a ModDetails/ModIface directly without any original Haskell source code, and running into a problem where GHC fails during Core generation with an internal panic. Also, note the warning (when GHC is compiled with -DDEBUG) that also only shows up when the instance is used:

Loading module Class
Registering module Class
Registering module Enums
Loading module A
Registering module A
WARNING:
  Glomming in A: [EeR :-> Once1]
  Call stack:
      CallStack (from HasCallStack):
        warnPprTrace, called at compiler/GHC/Core/Opt/OccurAnal.hs:85:5 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Opt.OccurAnal
hello: panic! (the 'impossible' happened)
  GHC version 9.3.20211130:
        lookupIdSubst

$fMyClassFoo
InScope {}
Call stack:
    CallStack (from HasCallStack):
      callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Utils.Panic
      pprPanic, called at compiler/GHC/Core/Subst.hs:260:17 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Subst
      lookupIdSubst, called at compiler/GHC/Core/SimpleOpt.hs:240:10 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.SimpleOpt
      simple_opt_expr, called at compiler/GHC/Core/SimpleOpt.hs:225:5 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.SimpleOpt

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

To reproduce:

1. Put attached Class.hs and A.hs files into input/ directory
2. Edit line 48 of Main.hs to point to your Hadrian stage1 directory
3. Run Main.hs and observe the error message