Possible bug in open type familes: Conflicting (a->a) and (a->a->a) instances

{-# LANGUAGE TypeFamilies #-}
module Test where
 
type family BoundsOf x
 
type instance BoundsOf (a->a) = Int
type instance BoundsOf (a->a->a) = (Int,Int)

This worked with GHC 7.6, but not with 7.8 HEAD (currently at 6cc7d3f1).

To check:

wget https://gist.github.com/nh2/6302087/raw/8167e7a1c8613aa384c2e8ca2f4ea9ade8745dc1/ghc-7.7-type-a-a-a-families.hs
ghci ghc-7.7-type-a-a-a-families.hs  # 7.6, all fine
ghci ghc-7.7-type-a-a-a-families.hs  # 7.7, breaks

On #ghc, we don't really understand whether this is the right thing to happen or not.

<rwbarton> see http://comments.gmane.org/gmane.comp.lang.haskell.glasgow.user/23734
<carter> c_wraith: im not sure why thats NOT working for open type familes too though
<carter> a->a and a->a->a don't overlap...
<rwbarton> ah it's in that thread. "Open (normal, old-fashioned) type families are essentially unchanged. In particular, coincident overlap and non-linear patterns *are* allowed.  The overlap check between open type family instances now does a unification without an "occurs check" to mark (x, x) and ([y], y) as overlapping, as necessary for type soundness."

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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