[GHC] #16095: Infinite loop during error reporting (unkillable, OOM)

25 Dec 2018

      #16095: Infinite loop during error reporting (unkillable, OOM)
-------------------------------------+-------------------------------------
           Reporter:  _deepfire      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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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 Compiling the repro snippet produces the following incomplete output and
 hangs GHC:
 {{{
 $ ghc repro.hs
 [1 of 1] Compiling Main             ( repro.hs, repro.o )

 repro.hs:16:22: error:
 }}}

 The GHC process then ignores Ctrl-C -- so it must be killed with SIGKILL.

 This minimal snippet depends on `generics-sop` (tested with version
 `0.4.0.0`).
 Sadly I didn't find a constraint in `base` to cause this behavior..

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

 import Generics.SOP (HasDatatypeInfo)

 data family   TF i a :: *
 data instance TF i a = R

 class C i a where
   method :: TF i a

 instance C i () where

 instance HasDatatypeInfo a => C i a where
   method = undefined function

 function :: C i a => TF i a
 function = method

 main = undefined
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16095
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