Ticket #3244: PanicRepro.hs

{-# OPTIONS_GHC -XTypeFamilies #-}
module PanicRepro where

-- Set up a type class with an associated type

class Config s where type T s

data X = UnusedXConstructor

instance Config X where type T X = String

-- Define two data constructors

data Container s = Container (T s)

data Action a = Action (IO a)

-- Cause a compiler panic

foo :: Action (Container X)
foo =
    let f _ = -- Note the constraint (String ~ T X) in definition of g.
              let g = return (Container "")
              in  Action g
    in f ()