[GHC] #14477: the 'impossible' happened, initTc: unsolved constraints

[GHC]

#14477: the 'impossible' happened, initTc: unsolved constraints
---------------------------------+--------------------------------------
        Reporter:  bmusin        |                Owner:  (none)
            Type:  bug           |               Status:  new
        Priority:  high          |            Milestone:
       Component:  Compiler      |              Version:  8.0.2
      Resolution:                |             Keywords:
Operating System:  Linux         |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
---------------------------------+--------------------------------------

Old description:



New description:

 Compile simple program using stack: stack ghc Main.hs

 code:
 import Control.Monad.Writer.Strict

 logNumber :: Int -> Writer [String] Int
 logNumber x = Writer (x, ["Got number: " ++ show x])

 multWithLog :: Writer [String] Int
 multWithLog = do
   a <- logNumber 3
   b <- logNumber 5
   tell ["Gonnay multiply these two"]
   return (a*b)

 [1 of 1] Compiling Main             ( Main.hs, Main.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initTc: unsolved constraints
   WC {wc_insol = [W] Writer_a19y :: t_a19x[tau:1] (CHoleCan: Writer)}

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

--

Comment (by bmusin):

 Completed ticked description, initially submitted incomplete bug by
 mistake.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14477#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler