Can't call rank-2 function with type family constraint

The following code compiles with 7.10.3:

newtype Bar r = Pow r deriving (Eq)
instance (Foo r) => Foo (Bar r)

type family Ctx a where Ctx t = (Foo (Bar t), Eq (Bar t))

run :: (forall t . (Ctx t) => t -> Int) -> Int
run g = undefined

foo :: (Foo (Bar t)) => t -> Int
foo = undefined

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

but fails in 8.0.1 with

    • Could not deduce (Foo t) arising from a use of ‘foo’
      from the context: Ctx t
        bound by a type expected by the context:
                   Ctx t => t -> Int
        at Main.hs:19:16-22
      Possible fix:
        add (Foo t) to the context of
          a type expected by the context:
            Ctx t => t -> Int
    • In the first argument of ‘run’, namely ‘foo’
      In the second argument of ‘($)’, namely ‘run foo’
      In the expression: print $ run foo

This code seems valid to me: when I call run foo, foo is specialized to (Foo (Bar t), Eq (Bar t)) => t -> Int), which meets the requirement of the argument to run.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture