Monomoprhic code makes ImpredicativeTypes infer an existential type

I realise that ImpredicativeTypes is a problematic extension, but I have found something that looks like an outright bug -- no polymorphism involved:

{-# LANGUAGE ImpredicativeTypes #-}

module Foo where

foo :: IO (Maybe Int)
foo = do
  pure $ case undefined :: Maybe String of
            Nothing
              -> Nothing
            Just _
              -> (undefined :: Maybe Int)

Produces the following errors:

foo.hs:7:3: error:
    • Couldn't match type ‘forall a. Maybe a’ with ‘Maybe Int’
      Expected type: IO (Maybe Int)
        Actual type: IO (forall a. Maybe a)
    • In a stmt of a 'do' block:
        pure
        $ case undefined :: Maybe String of {
            Nothing -> Nothing
            Just _ -> (undefined :: Maybe Int) }
      In the expression:
        do { pure
             $ case undefined :: Maybe String of {
                 Nothing -> Nothing
                 Just _ -> (undefined :: Maybe Int) } }
      In an equation for ‘foo’:
          foo
            = do { pure
                   $ case undefined :: Maybe String of {
                       Nothing -> Nothing
                       Just _ -> (undefined :: Maybe Int) } }

foo.hs:11:19: error:
    • Couldn't match type ‘a’ with ‘Int’
      ‘a’ is a rigid type variable bound by
        a type expected by the context:
          forall a. Maybe a
        at foo.hs:11:19
      Expected type: forall a. Maybe a
        Actual type: Maybe Int
    • In the expression: (undefined :: Maybe Int)
      In a case alternative: Just _ -> (undefined :: Maybe Int)
      In the second argument of ‘($)’, namely
        ‘case undefined :: Maybe String of {
           Nothing -> Nothing
           Just _ -> (undefined :: Maybe Int) }’

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	A.SerranoMena@uu.nl, simonpj@microsoft.com, Alejandro, Jones, Mena, Peyton, Serrano, Simon
Operating system
Architecture