Superclass functional dependencies are ignored during instance selection

The following example compiles with ghc-6.12.3, but not with ghc-7.0.0.20100924:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

module Ambiguity where

class C1 a b | b -> a
class (C1 a b) => C2 a b where
  foo :: b -> b

data A = A
data B = B
instance C1 A B
instance C2 A B

-- this is accepted by both 6.12.3 and 7
runFoo :: C2 a b => b -> b
runFoo = foo

-- this is accepted by 6.12.3, but not by 7
runFoo2 :: B -> B
runFoo2 = foo

A straightforward fix is to add the b -> a fundep to the C2 class as well.

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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