Type unsoundness with type families and UndecidableInstances

''This problem has already been fixed but I'm creating a ticket here to attach an example that demonstrates the problem, following a suggestion here: https://mail.haskell.org/pipermail/glasgow-haskell-users/2013-August/024194.html''

The following is accepted by GHC 7.6.

type family F a
type instance F (a -> a) = Int
type instance F (a -> a -> a) = IO String

However these definitions allows an Int to be coerced to IO String under !UndecidableInstances, breaking type safety. The problem is that you can define an infinite type LA = LA -> LA using type families, then F LA can be reduced to both Int and to IO String.

I'll attach a program that demonstrates how this problem can cause a segfault. https://github.com/takano-akio/type-family-overlap

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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