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The following program compiles (with -O) on GHC 7.0.4 through 8.6.3, but panics on HEAD:

{-# LANGUAGE DeriveFunctor #-}
module Bug where

data V2 a = V2 !a !a deriving Functor

(*^) :: (Functor f, Num a) => a -> f a -> f a
(*^) a = fmap (a*)

(*!!) :: (Functor m, Functor r, Num a) => a -> m (r a) -> m (r a)
s *!! m = fmap (s *^) m

type M22 a = V2 (V2 a)

det22 :: Num a => M22 a -> a
det22 (V2 (V2 a b) (V2 c d)) = a * d - b * c

inv22 :: Fractional a => M22 a -> M22 a
inv22 m@(V2 (V2 a b) (V2 c d)) = (1 / det) *!! V2 (V2 d (-b)) (V2 (-c) a)
  where det = det22 m

$ ~/Software/ghc4/inplace/bin/ghc-stage2 -fforce-recomp Bug.hs -O
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20190219 for x86_64-unknown-linux):
        tyConAppArgs
  a_a1FK
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1159:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1183:50 in ghc:Type

Note that this panic does not occur if V2 is defined without !s.

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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