[GHC] #12083: ghc-8.0.1-rc4: tyConRoles sees a TcTyCon

[GHC]

#12083: ghc-8.0.1-rc4: tyConRoles sees a TcTyCon
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           Reporter:  _deepfire      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc4
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:  Yes.           |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UnicodeSyntax #-}

 import           Prelude.Unicode

 type Constrd a = Num a ⇒ a

 data ADT a = ADT (Constrd a) ExistentiallyLost

 data ExistentiallyLost = ∀ u. TC u ⇒ ExistentiallyLost u

 class u ~ (ATF1 u, ATF2 u) ⇒ TC u where
     type ATF1    u ∷ *
     type ATF2    u ∷ *
     uie_handlers   ∷ ADT Int

 -- Loop:
 --  - ADT depends on ExistentiallyLost (also the Constrd appendage)
 --  - ExistentiallyLost depends on TC
 --  - TC depends on ADT
 }}}
 -->
 {{{

 [1 of 1] Compiling Main             ( /home/deepfire/src/ghc-testcases
 /tyconroles-sees-a-tctycon-tyalias.hs, interpreted )
 <- ghc: panic! (the 'impossible' happened)
      (GHC version 8.0.0.20160421 for x86_64-unknown-linux):
         tyConRoles sees a TcTyCon Constrd

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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12083>
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