Plugin breakage in combination with Aeson

[sorki]

Some weird interaction with Generics or ToJSON results in GHC panic.

Reproducer sorki/polytype@abc11ab

Complete file https://github.com/sorki/polytype/blob/abc11abbd00528b50a5e7f6fd7593fb2d562b059/src/Polytype/Asciinema.hs

Edit: GHC 8.10.1

[tek]

For searchability, because I just spent quite some time on this:

Error:

<no location info>: error:                            
    ghc: panic! (the 'impossible' happened)                                                                  
  (GHC version 8.10.1:                                                                                       
        mergeSATInfo                                        
  Left:STSVSVSVSVSV, Right:STNSSVSCSVSC                                                                      
  Call stack:                                               
      CallStack (from HasCallStack):                                                                         
        callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable                                  
        pprPanic, called at compiler/simplCore/SAT.hs:152:20 in ghc:SAT    

Workaround so you don't have to deactivate the plugin project-wide:

 {-# OPTIONS_GHC -fclear-plugins #-}

[tek]

FTR, I noticed that only get panics in my projects with -O2.

so I use this:

when:
  - condition: 'impl(ghc < 8.10)'
    ghc-options: -O2

[Anrock]

GHC ticket seems to be related: https://gitlab.haskell.org/ghc/ghc/-/issues/14649, fendor provided minimal repro with aeson there. I'm hitting similar panic, but not using aeson in my project and I don't have minimal repro.