[GHC] #14458: ghc: panic! -- initTc: unsolved constraints

[GHC]

#14458: ghc: panic! -- initTc: unsolved constraints
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           Reporter:  lijero         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 So this is an interesting bug. If I mistype parse' as parse, /and/ do not
 have main defined, the error occurs, but not with either individual one.

 I'm sure the code sucks, but I wasn't intending for this to end up
 anywhere, it just happened to trigger the crash.

 {{{
 lijero at desktop:~/code/ab$ ghc AB.hs
 [1 of 1] Compiling Main             ( AB.hs, AB.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initTc: unsolved constraints
   WC {wc_insol =
         [W] parse_ay8 :: t_ay7[tau:1] (CHoleCan: parse)
         [W] parse_ayp :: t_ayo[tau:1] (CHoleCan: parse)}

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

 {{{#!hs
 module Main where

 data Token = KUnit
            | KApp
            | KLam
            | KVar String

 data Ast  = AUnit
           | AApp Ast Ast
           | ALam Ast Ast
           | AVar String

 parse' :: [Token] -> ([Token] -> Ast -> Maybe Ast) -> Maybe Ast
 parse' (KUnit:xs) fail = fail xs AUnit
 -- GHC does not crash when parse is correctly written parse' in both of
 the below instances
 parse' (KApp:xs) fail =
   parse' xs (\ xs' f -> AApp f <$> parse xs' fail)
 parse' (KLam:xs) fail =
   parse' xs (\ xs' a -> ALam a <$> parse xs' fail)
 parse' (KVar n:xs) fail = fail xs (AVar n)

 -- GHC does not crash when main is defined
 --main :: IO ()
 --main = putStrLn ""
 }}}

 With main defined, GHC correctly reports "variable not in scope".
 With parse corrected to parse', GHC correctly reports that main is not
 defined.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14458>
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