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GHC panics when the following program is loaded in ghci:

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}

module PatternPanic
    (
      Outer(..),
      Inner(..),
      pattern TestPat
    ) where

data Outer = Outer Inner Inner
data Inner = Inner Int Int


pattern TestPat :: String -> String -> Outer
pattern TestPat ref1 ref2 <-
    Outer (Inner (extractRef -> Just ref1)
                 (extractRef -> Just ref2)
          )
          (Inner (isSameRef ref1 -> True)
                 (isSameRef ref2 -> True)
          )

------------ Helper functions ------------

extractRef :: Int -> Maybe String
extractRef 0 = Nothing
extractRef v = Just $ show v


isSameRef :: String -> Int -> Bool
isSameRef refName e
    | Just refName2 <- extractRef e
    = refName == refName2
isSameRef _ _ = False

-- Note that the GHC panic does not happen if 'isSameRef' is replaced with the following version.
-- isSameRef :: String -> Int -> Bool
-- isSameRef refName e =
--     extractRef e == Just refName

Returns

GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
Prelude> :l PatternPanic.hs
[1 of 1] Compiling PatternPanic     ( PatternPanic.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.3 for x86_64-apple-darwin):
	isUnliftedType
  r_a2Qa :: TYPE rep_a2Q9
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:2021:10 in ghc:Type

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

This simplified code snippet was extracted from a relatively large codebase (~180k LOC). Note, that we would get the same error when we try to compile this application with -O2 with ghc > 8.4. Unfortunately, I am not able to reproduce compilation failure with this minimal example, though.

I tried it with GHC 8.9.20190323 x86_64-apple-darwin - same error.

#14828 (closed) looks quite similar.

pattern-panic.zip