[GHC] #11675: Monomoprhic code makes ImpredicativeTypes infer an existential type

[GHC]

#11675: Monomoprhic code makes ImpredicativeTypes infer an existential type
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           Reporter:  _deepfire      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I realise that ImpredicativeTypes is a problematic extension, but I have
 found something that looks like an outright bug -- no polymorphism
 involved:

 {{{#!hs
 {-# 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) }’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11675>
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