[GHC] #15026: Type deduction panic

[GHC]

#15026: Type deduction panic
-------------------------------------+-------------------------------------
           Reporter:  moomin         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Windows
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!lhs
 > {-# LANGUAGE DeriveFunctor #-}

 > module Bug where

 > import qualified Data.Set as S

 > import Data.Foldable (fold, foldMap, toList)
 > import Data.List.Split (splitOn)
 > import Data.Monoid -- ((<>), Sum, mempty, mappend)
 > import Data.Functor.Foldable

 > newtype Component = Component (Int,Int) deriving (Eq, Ord, Show)

 > data Day24State v a = Day24State {
 >   valueToMatch :: Int,
 >   componentsToUse :: S.Set Component,
 >   componentMetric :: v,
 >   nextStates :: [a]
 > } deriving (Functor, Show)

 > type Day24StateFix v a = Fix (Day24State v)
 }}}

 Now in GHCI type

 > :t Data.Functor.Foldable.ana f (undefined :: Day24StateFix Component)

 Produces

 ghc.EXE: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-mingw32):
         initTc: unsolved constraints
   WC {wc_insol = [W] f_akbJ :: t_akbI[tau:3] (CHoleCan: f)}

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

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15026>
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