[GHC] #11458: Terrible failure of type inference in visible type application

[GHC]

#11458: Terrible failure of type inference in visible type application
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider
 {{{
 -- optIntArg :: (Maybe Int -> t2) -> (t2,t2)
 optIntArg f = (f Nothing, f (Just True))
 }}}
 This is rejected (by HEAD)
 {{{
 T11379a.hs:5:30: error:
     * Couldn't match type `a' with `Bool'
       `a' is a rigid type variable bound by
         a type expected by the context:
           forall a. Maybe a
         at T11379a.hs:5:30
       Expected type: forall a. Maybe a
         Actual type: Maybe Bool
     * In the first argument of `f', namely `(Just True)'
       In the expression: f (Just True)
       In the expression: (f Nothing, f (Just True))
 }}}
 but if you put the tuple components the other way round, it works fine
 {{{
 optIntArg f = (f (Just True), f Nothing)
 }}}
 Adding the commented-out signature also makes it work fine.

 I'm almost certain that this is caused by visible type application;
 perhaps `Nothing` gets delayed instantiation, and then `f`'s type becomes
 `forall a. Maybe a`.  Utterly bogus.

 I suspect it'll be fixed by Richards `ReturnTv` work, but let's make sure
 it is.  We can't release this!!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11458>
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