[GHC] #13659: Bug report: "AThing evaluated unexpectedly tcTyVar a_alF"

[GHC]

#13659: Bug report: "AThing evaluated unexpectedly tcTyVar a_alF"
-------------------------------------+-------------------------------------
           Reporter:  costaparas     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following code:

 {{{#!hs
 {-# LANGUAGE GADTs, EmptyDataDecls, TypeFamilies, TypeOperators,
 DataKinds, FlexibleInstances #-}

 {- Defines a C-like printf function using DataKinds extensions. -}

 module Printf where

 -- format string parameterized by a list of types
 data Format (fmt :: [*]) where
   X :: Format '[]                   -- empty string, i.e. ""
   L :: a -> String -> Format '[]    -- string literal, e.g. "hello"
   S :: a -> Format '[String]        -- "%s"
   I :: Format a -> Format '[Int, a] -- "%d"
 }}}

 produces the following error:

 GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Printf           ( Printf.hs, interpreted )

 a.hs:12:27:ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         AThing evaluated unexpectedly tcTyVar a_alF

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

 when run using **ghci Printf.hs**

 on Liunx Ubuntu 16.04 LTS 64-bit with Intel® Core™ i7-7500U CPU @ 2.70GHz

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13659>
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