[GHC] #14893: GHC hangs while producing error message for incorrect TypeFamilies application

5 Mar 2018

      #14893: GHC hangs while producing error message for incorrect TypeFamilies
application
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           Reporter:  coopercm       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Linux
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Here's a minimally reproducible example:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE MultiParamTypeClasses #-}

 module Main where

 newtype Token f a = Token { unToken :: f a }

 class Class x (f :: * -> *) where
   type Meta x f :: *
   type Body x f :: *
   type Body x f = Token f Meta

 main :: IO ()
 main = return ()

 }}}

 This hangs like so
 {{{
 [2 of 2] Compiling Main             ( app/Main.hs, .stack-
 work/dist/x86_64-linux/Cabal-2.0.1.0/build/foo-exe/foo-exe-tmp/Main.o )

 }}}

 I tested on 8.2.2 and 8.0.2 and got the same behavior.

 The closest existing bug report I could find was
 https://ghc.haskell.org/trac/ghc/ticket/12386.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14893
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