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$ ghci -ignore-dot-ghci
GHCi, version 8.3.20171208: http://www.haskell.org/ghc/  :? for help
Prelude> import Data.Kind
Prelude Data.Kind> :set -XPolyKinds -XDataKinds -XTypeInType -XTypeOperators 
Prelude Data.Kind> type Cat ob = ob -> ob -> Type
Prelude Data.Kind> data ISO' :: Cat i -> i -> i -> Type
Prelude Data.Kind> type ISO cat a b = ISO' cat a b -> Type
Prelude Data.Kind> type (a <--> b) iso cat = ISO (iso :: cat a b)
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.3.20171208 for x86_64-unknown-linux):
	piResultTy
  k_a1x1[tau:1]
  a_a1wU[sk:1]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1144:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:951:35 in ghc:Type

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

Prelude Data.Kind> 

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

b) iso cat = ISO (iso :: cat a b)\r\nghc-stage2: panic! (the 'impossible' happened)\r\n (GHC version 8.3.20171208 for x86_64-unknown-linux):\r\n\tpiResultTy\r\n k_a1x1[tau:1]\r\n a_a1wU[sk:1]\r\n Call stack:\r\n CallStack (from HasCallStack):\r\n callStackDoc, called at compiler/utils/Outputable.hs:1144:37 in ghc:Outputable\r\n pprPanic, called at compiler/types/Type.hs:951:35 in ghc:Type\r\n\r\nPlease report this as a GHC bug: http://www.haskell.org/ghc/reportabug\r\n\r\nPrelude Data.Kind> \r\n}}}","type_of_failure":"OtherFailure","blocking":[]} -->