Type checker rejects valid program

Since at least 6e280c2c GHC fails to compile resourcet due to a likely erroneous type error.

Here is a minimal case that reproduces the error,

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FlexibleContexts #-}

module Hi where

import Control.Monad (liftM, ap)
data Allocated a = Allocated a

newtype Acquire a = Acquire ((forall b. IO b -> IO b) -> IO (Allocated a))

instance Functor Acquire where
    fmap = liftM

instance Applicative Acquire where
    pure a = Acquire (\_ -> return (Allocated a))
    (<*>) = ap

instance Monad Acquire where
    return = pure
    Acquire f >>= g' = Acquire $ \restore -> do
        Allocated x <- f restore
        let Acquire g = g' x
        Allocated y <- g restore
        return $! Allocated y

This fails with,

$ ghc Hi.hs
[1 of 1] Compiling Hi               ( Hi.hs, Hi.o )

Hi.hs:22:21: error:
    • Couldn't match expected type ‘t’
                  with actual type ‘(forall b1. IO b1 -> IO b1) -> IO (Allocated b)’
      ‘t’ is a rigid type variable bound by
        the inferred type of g :: t at Hi.hs:22:13-28
    • In the pattern: Acquire g
      In a pattern binding: Acquire g = g' x
      In the expression:
        do { Allocated x <- f restore;
             let Acquire g = g' x;
             Allocated y <- g restore;
             return $! Allocated y }
    • Relevant bindings include
        g' :: a -> Acquire b (bound at Hi.hs:20:19)
        (>>=) :: Acquire a -> (a -> Acquire b) -> Acquire b
          (bound at Hi.hs:20:15)

Despite compiling with 8.0.1 and earlier versions.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonpj
Operating system
Architecture