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Spurious potential superclass cycle with constraint synonyms

    • Closed Issue created by Edward Z. Yang @ezyang

    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.

    Trac metadata
    Trac field Value
    Version 7.11
    Type Bug
    TypeOfFailure OtherFailure
    Priority high
    Resolution Unresolved
    Component Compiler (Type checker)
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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