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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
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