[GHC] #12055: Typechecker panic instead of proper error

[GHC]

#12055: Typechecker panic instead of proper error
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Well HEAD for this gives
 {{{
 T12055.hs:18:1: error:
     • Non type-variable argument in the constraint: Category (Dom f)
       (Use FlexibleContexts to permit this)
     • In the context: (Category (Dom f), Category (Cod f))
       While checking the super-classes of class ‘Functor’
       In the class declaration for ‘Functor’

 T12055.hs:26:1: error:
     • Fundeps in class ‘Fun’
       (Use FunctionalDependencies to allow fundeps)
     • In the class declaration for ‘Fun’

 T12055.hs:29:5: error:
     • Illegal instance declaration for ‘Fun p q f’
         (All instance types must be of the form (T a1 ... an)
          where a1 ... an are *distinct type variables*,
          and each type variable appears at most once in the instance head.
          Use FlexibleInstances if you want to disable this.)
     • In the instance declaration for
         ‘Fun (p :: Cat j) (q :: Cat k) (f :: j -> k)’
 }}}
 And if you add the missing language extensions
 {{{
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE UndecidableInstances #-}
 }}}
 then it compiles cleanly.

 I suppose someone could bisect their way to the patch that fixed HEAD. Or
 we could just leave it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12055#comment:2>
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