[GHC] #15600: Bug while writing a simple parser with Parsec

[GHC]

#15600: Bug while writing a simple parser with Parsec
-------------------------------------+-------------------------------------
           Reporter:  roehst         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:  x86_64         |   Type of failure:  None/Unknown
  (amd64)                            |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 module Main where

 import Control.Monad.Identity
 import Control.Monad.State
 import Data.List
 import Text.ParserCombinators.Parsec

 type Name = String

 data Value = VlInt Int | VlBool Bool deriving Show

 data Term =
   TmVal Value
   | TmVar Name
   | TmLet Name Term Term
   | TmAdd Term Term
   deriving Show

 parseTerm :: Parser Term
 parseTerm = parseVal <|> parseVar <|> parseLet <|> parseAdd
 }}}

 And then I get:

 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( Main.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initTc: unsolved constraints
   WC {wc_insol =
         [W] parseVal_a2YR :: t_a2YQ[tau:1] (CHoleCan: parseVal)
         [W] parseVar_a2YY :: t_a2YX[tau:1] (CHoleCan: parseVar)
         [W] parseLet_a2Z5 :: t_a2Z4[tau:1] (CHoleCan: parseLet)
         [W] parseAdd_a2Z8 :: t_a2Z7[tau:1] (CHoleCan: parseAdd)}

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15600>
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