core lint error with arrow notation and GADTs

[benmos]

The following code panics GHC (with 7.0.3, 7.2 and 7.4.0.20111219):

{-# LANGUAGE Arrows, GADTs #-}
import Control.Arrow

data Value a where BoolVal :: Value Bool

class ArrowInit f where
    arrif :: f b -> ()

instance ArrowInit Value where
    arrif = proc BoolVal -> returnA -< ()
    -- arrif = arr (\BoolVal -> ())

I am attaching the -dcore-lint from 7.4.

[benmos]

Core Lint output

[benmos]

Source file

[ross]

Simplified version, avoiding classes:

{-# LANGUAGE Arrows, GADTs #-}
module ArrowBug5777 where

import Control.Arrow

data Value a where BoolVal :: Value Bool

arrif :: Value Bool -> ()
arrif = proc BoolVal -> returnA -< ()

[benmos]

The workaround is to move the GADT data constructor out of the 'proc' pattern and into a 'let':

data Value a where BoolVal :: Int -> Value Bool

arrif :: Value Bool -> Int
arrif = proc x -> do
          let BoolVal i = x
          returnA -< i