[GHC] #11278: Spurious potential superclass cycle with constraint synonyms

22 Dec 2015

      #11278: Spurious potential superclass cycle with constraint synonyms
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           Reporter:  ezyang         |             Owner:  simonpj
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.11
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The superclass cycle detector isn't smart enough to deal with constraint
 synonyms:

 {{{
 {-# LANGUAGE ConstraintKinds #-}
 module A where
 type K a = (Show a, Read a)
 class K a => C a where
 }}}

 On HEAD I get:

 {{{
 A.hs:4:1: error:
     • Potential superclass cycle for ‘C’
         one of whose superclasses is ‘GHC.Classes.(%,%)’
         one of whose superclass constraints is headed by a type variable:
           ‘c1’
       Use UndecidableSuperClasses to accept this
     • In the class declaration for ‘C’
 }}}

 The error goes away if the constraint synonym only has one constraint in
 it.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11278
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