[GHC] #13487: GHC panic with deferred custom type errors

[GHC]

#13487: GHC panic with deferred custom type errors
-------------------------------------+-------------------------------------
        Reporter:  DimaSamoz         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  GHCi              |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * os:  MacOS X => Unknown/Multiple
 * component:  Compiler => GHCi
 * milestone:   => 8.4.1


Comment:

 Thanks for the bug report! Here is a minimal file which exhibits the
 issue:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Mezzo where

 import Data.Kind (Constraint)
 import GHC.TypeLits

 data Foo a b where
   (:-:) :: Error a b => a -> b -> Foo a b

 type family Error a b :: Constraint where
   Error Int Int = ()
   Error _   _   = TypeError ('Text "GHC panic in 3... 2... 1...")
 }}}

 Now load this into GHCi like so:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Mezzo.hs -fdefer-type-errors
 GHCi, version 8.2.0.20170321: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Mezzo            ( Mezzo.hs, interpreted )
 Ok, modules loaded: Mezzo.
 λ> let v = 'a' :-: 'b'

 <interactive>:1:9: warning: [-Wdeferred-type-errors]
     • GHC panic in 3... 2... 1...
     • In the expression: 'a' :-: 'b'
       In an equation for ‘v’: v = 'a' :-: 'b'
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170321 for x86_64-unknown-linux):
         nameModule
   system irred_a2AK
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1191:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Name.hs:239:3 in ghc:Name
 }}}

 I can reproduce this with 8.0.1, 8.0.2, 8.2.1, and HEAD.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13487#comment:1>
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