Type error, but all type error messages are suppressed: GHC 9.6.6, 9.8.2, 9.10.1

Summary

I was implementing some Data types a la carte stuff and encountered a ghc panic. The steps to reproduce contain a minimal verison (that I found) reproducing the issue. 9.6.6, 9.8.2, 9.10.1 all panicked.

Steps to reproduce

Compile the following program with ghc using any of the three aforementioned versions.

If I remove the unused parameter f from Buggy at the declaration and in the Ghcbug typeclass then the code compiles without issues.

module Bug where

import Control.Monad.State

data (f :+: g) a = Inl (f a) | Inr (g a)
    deriving Functor

newtype Buggy f m = Buggy { thing :: m Int }

class GhcBug f where
  demo :: MonadState (Buggy f m) m => f (m Int) -> m Int

instance (GhcBug f, GhcBug g) => GhcBug (f :+: g) where
    demo (Inl l) = demo l
    demo (Inr r) = demo r

Error messages

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.6.6:
        lookupIdSubst
  $dMonadState_aLh
  InScope {f_aKX g_aKY $dGhcBug_aKZ $dGhcBug_aL0 m_aL5
           $dMonadState_aL6}
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:189:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Subst.hs:197:17 in ghc:GHC.Core.Subst
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:454:29 in ghc:GHC.Utils.Error


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

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.8.2:
        lookupIdSubst
  $dMonadState_aFd
  InScope {f_aER g_aES $dGhcBug_aET $dGhcBug_aEU m_aEZ
           $dMonadState_aF0}
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:191:37 in ghc-9.8.2-inplace:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Subst.hs:197:17 in ghc-9.8.2-inplace:GHC.Core.Subst
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:503:29 in ghc-9.8.2-inplace:GHC.Utils.Error


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

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.10.1:
        lookupIdSubst
  $dMonadState_aFt
  InScope {f_aF6 g_aF7 $dGhcBug_aF8 $dGhcBug_aF9 m_aFf
           $dMonadState_aFg}
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:190:37 in ghc-9.10.1-inplace:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Core/Subst.hs:196:17 in ghc-9.10.1-inplace:GHC.Core.Subst
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:507:29 in ghc-9.10.1-inplace:GHC.Utils.Error


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

Expected behavior

Ghc should not panic.

Environment

• Architecture: x86_64
• Operating System: NixOS 24.11.20240926.1925c60
• GHC version: 9.6.6, 9.8.2, 9.10.1

Related issue

Issue #25064 (closed) seems to be related.