Recursive type family causes <<loop>>

This file:

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

data Proxy a = Proxy

foo :: Proxy (F (Fix Id)) -> ()
foo = undefined

newtype Fix a = Fix (a (Fix a))
newtype Id a = Id a

type family F a
type instance F (Fix a) = F (a (Fix a))
type instance F (Id a) = F a

main :: IO ()
main = print $ foo Proxy

Dies with <<loop>>. The type family is recursive, of course:

*Main> :kind! F (Fix Id)
F (Fix Id) :: *^CInterrupted.

But <<loop>> is still not the behavior I'd expect. The actual value is just undefined.

In the file that this example came up, the situation was even worse -- there was a situation where

moldMapOf l f = runAccessor . l (Accessor . f)
main = print $ (flip appEndo [] . moldMapOf (ix 3) (Endo . (:)) $ testVal :: [Int]) -- <<loop>>
main = print $ (flip appEndo [] . runAccessor . (ix 3) (Accessor . Endo . (:)) $ testVal :: [Int]) -- undefined

I.e. substitution can turn one program (which happens to be ⊥ here, admittedly, but that's not fundamental) into another (<<loop>>). This makes it very tricky to track down the recursive type family. If necessary I can hunt down a working test case and post it here -- it's a bit tricky to get working, though.

Trac metadata

Trac field	Value
Version	7.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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