Spurious potential superclass cycle with constraint synonyms

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
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Operating system
Architecture