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Consider my two instance declarations. The second one will hang ghc. If I change (coerce) to (coerce :: Normal a -> Sized a) it compiles fine. The first declaration also works fine.

{-# LANGUAGE TypeFamilies, FlexibleInstances #-}

import qualified Data.FingerTree as FT
import GHC.Exts

class DOps a where
  plus :: a -> D a -> a

type family D a :: *
type instance D (FT.FingerTree (Size Int, v) (Sized a)) = [Diff (Normal a)]

type family Normal a :: *

data Diff a = Add Int a

newtype Sized a = Sized a
newtype Size a = Size a

-- This works:
instance (FT.Measured (Size Int, v) (Sized a), Coercible (Normal a) (Sized a)) => DOps (FT.FingerTree (Size Int, v) (Sized a)) where
  plus = foldr (\(Add index val) seq -> FT.singleton ((coerce) val))

-- This hangs:
instance (FT.Measured (Size Int, v) (Sized a), Coercible (Normal a) (Sized a)) => DOps (FT.FingerTree (Size Int, v) (Sized a)) where
  plus = foldr (flip f)
    where f seq x = case x of
            Add index val -> FT.singleton ((coerce) val)

$ ~/downloads/ghc-7.10.0.20150123/out/bin/ghci --version
The Glorious Glasgow Haskell Compilation System, version 7.10.0.20150123

Trac metadata

Trac field	Value
Version	7.10.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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