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

30 Sep 2018

      #15692: GHC panic from pattern synonyms + deferred type errors
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms, TypeInType        |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# Language DataKinds, TypeOperators, PolyKinds, PatternSynonyms, GADTs
 #-}

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

 import Data.Kind

 data Ctx :: Type -> Type where
  E     :: Ctx(Type)
  (:&:) :: a -> Ctx(as) -> Ctx(a -> as)

 data ApplyT (k::Type) :: k ->  Ctx(k) -> Type where
  AO :: a -> ApplyT(Type) a E
  AS :: ApplyT(ks)      (f a) ctx
     -> ApplyT(k -> ks) f     (a:&:ctx)

 pattern ASSO = AS (AS (AO False))
 }}}

 {{{
 $ ghci -ignore-dot-ghci 463.hs
 hs/463.hs:16:27: warning: [-Wdeferred-type-errors]
     • Couldn't match type ‘a a1 a2’ with ‘Bool’
       Expected type: a3
         Actual type: Bool
     • In the pattern: False
       In the pattern: AO False
       In the pattern: AS (AO False)
    |
 16 | pattern ASSO = AS (AS (AO False))
    |                           ^^^^^
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20180828 for x86_64-unknown-linux):
         urk! lookup local fingerprint
   $mASSO
   [iESflb :-> ($trModule, 1ca40dc83a9c879effdb760462cc9a2d),
    iESgKD :-> ($tc'E, 79f67a27a14dc1bb6eecb39e4b061e2c),
    iESgKF :-> ($tc':&:, 24793c0c1652ffcf92e04f47d38fa075),
    iESgKH :-> ($tcCtx, a3f9358cbfe161bf59e75500d70ce0ae),
    iESgKI :-> ($tc'AO, 72111d1891cb082e989c20a2191a8b4b),
    iESgKK :-> ($tc'AS, ff019c04c400d5fbdd46ff8a816d4913),
    iESgKM :-> ($tcApplyT, cbfe28374b4115925c7213e6330ab115)]
   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
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