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Here is a small program:

type family F a b
type instance F (Maybe a) b = b -> F a b

class C a where
  go :: (forall a. Maybe a -> b -> a) -> a -> F a b

instance C a => C (Maybe a) where
  go f a b = go f (f a b)

The current head complains:

    Could not deduce (F a b ~ F a b) from the context (C a)
    NB: `F' is a type function, and may not be injective
    Expected type: b -> F a b
      Actual type: F (Maybe a) b

1. 12.3 accepts it.

Trac metadata

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