[GHC] #15692: GHC panic from pattern synonyms + deferred type errors

[GHC]

#15692: GHC panic from pattern synonyms + deferred type errors
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  patsyn/should_fail/T15692
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 Is this the same issue?

 {{{#!hs
 {-# Language DataKinds, PatternSynonyms, KindSignatures, GADTs #-}

 {-# Options_GHC -fdefer-type-errors #-}

 import Data.Kind

 data N = O | S N

 data Fin :: N -> Type where
   FinO :: Fin (S n)

 data Exists :: (Type -> Type) -> Type where
   Exists :: f xx -> Exists f

 pattern O' = Exists FinO
 }}}

 {{{
 $ ghci -ignore-dot-ghci hs/486.hs
 GHCi, version 8.7.20180828: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( hs/486.hs, interpreted )

 hs/486.hs:15:21: warning: [-Wdeferred-type-errors]
     • Couldn't match kind ‘*’ with ‘N’
       When matching types
         a :: * -> *
         Fin :: N -> *
       Expected type: a xx
         Actual type: Fin a0
     • In the pattern: FinO
       In the pattern: Exists FinO
       In the declaration for pattern synonym ‘O'’
    |
 15 | pattern O' = Exists FinO
    |                     ^^^^
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20180828 for x86_64-unknown-linux):
         urk! lookup local fingerprint
   $mO'
   [iESflb :-> ($trModule, 1ca40dc83a9c879effdb760462cc9a2d),
    iESgex :-> ($tc'O, 111d9d0cf0cb57db9c77a4d216344e54),
    iESgKA :-> ($tc'S, 2d7f65aefd4d8c7deac332c17204e2c9),
    iESgKC :-> ($tcN, 04f8b57a4955bc680fd71fe4cee31a00),
    iESgKD :-> ($tc'FinO, a20e4215870bd6fb39afb23de637e84a),
    iESgKF :-> ($tcFin, 8a38d4451422cc87d0cc0459132dad73),
    iESgKG :-> ($tc'Exists, 365c3f6bd1b8c9580d1ae883b83c2c68),
    iESgKI :-> ($tcExists, 11a84f66f5d99fe1d3547c0ad0f538ae)]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/iface/MkIface.hs:524:37 in
 ghc:MkIface

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

 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15692#comment:6>
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