Using this file (both with the itask and the master compiler):

module test

import StdEnv
import Data.Maybe
import Data.Functor
import Control.Applicative

class expr v where
	lit :: i -> v i | toString i
	(+.) infixl 6 :: (v i) (v i) -> v i | + i

instance expr Maybe where
	lit i = Just i
	+. x y = (+) <$> x <*> y

:: Print a = Print String
print :: (Print a) -> String
print (Print a) = a
instance expr Print where
	lit i = Print (toString i)
	+. (Print l) (Print r) = Print (l +++ "+" +++ r)

printEval :: (A.v: v a | expr v) -> (Maybe a, String)
printEval f = (f, let (Print p) = f in p)

Start :: (Maybe Int, String)
Start = printEval (lit 4 +. lit 38)

This works fine and gives the output:

((Just 42),"4+38")

However, if we change

:: Print a = Print a

to

:: Print a =: Print a

The output is wrong, and when we use the second element it obviously crashes:

((Just 42),_f54;54.12)

If we change

printEval f = (f, let (Print p) = f in p)

to

printEval f = (f, print f)

It works again.

The construction let (Print p) = f in p works fine in a non rank2 polymorphic function