[GHC] #11122: Ambiguous inferred type causes a panic

[GHC]

#11122: Ambiguous inferred type causes a panic
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           Reporter:  tuplanolla     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Linux
       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 the following program.

 {{{#!hs
 {-# LANGUAGE PartialTypeSignatures #-}

 module Main where

 import Text.Parsec
 import Text.Parsec.String

 parser :: Parser _
 parser = read <$> many digit

 data Wrapper = Wrapper Int

 wrapperParser = Wrapper <$> parser
 }}}

 I am not sure whether it is valid or not, but it breaks the type checker
 as follows.

 {{{#!hs
 Program.hs:9:1:
     No instance for (Read w_)
     When checking that ‘parser’ has the specified type
       parser :: forall w_. Parser w_
     Probable cause: the inferred type is ambiguous

 Program.hs:13:29:
     Couldn't match type ‘w_’ with ‘Int’
       ‘w_’ is untouchable
         inside the constraints ()
         bound by the inferred type of
                  wrapperParser :: ParsecT
                                     String ()
 Data.Functor.Identity.Identity Wrapper
         at Program.hs:13:1-34ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for x86_64-unknown-linux):
         No skolem info: w__avnz[sk]
 }}}

 Disabling `PartialTypeSignatures` makes no difference.
 Switching from `parsec-3.1.9` to `megaparsec-4.1.1` and changing `many
 digit` to `some digitChar` does not have an effect either.
 Replacing `_` with `Int` seems to be the only obvious way to make the
 problem disappear.
 Alas that defeats the point of using partial type signatures or typed
 holes in the first place.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11122>
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