[GHC] #13510: GHC panic with -fdefer-type-errors

[GHC]

#13510: GHC panic with -fdefer-type-errors
-------------------------------------+-------------------------------------
           Reporter:  chrismwendt    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           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:
-------------------------------------+-------------------------------------
 This program causes GHCi to panic:

 {{{
 import Data.Monoid

 a = ""

 x :: () -> Int
 x b = a <> b
 }}}

 {{{
 $ stack --version
 Version 1.3.2 x86_64 hpack-0.15.0
 $ stack exec ghci program.hs
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /Users/chrismwendt/.ghci
 [1 of 1] Compiling Main             ( program.hs, interpreted )

 program.hs:6:12: warning: [-Wdeferred-type-errors]
     • Couldn't match expected type ‘Int’ with actual type ‘()’
     • In the second argument of ‘(<>)’, namely ‘b’
       In the expression: a <> b
       In an equation for ‘x’: x b = a <> b
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-apple-darwin):
         corePrepPgm
   [False] cobox_r1MZ = typeError
                          @ 'VoidRep
                          @ (Int :: *) ~# (() :: *)
                          "program.hs:6:12: error:\n\
                          \    \\226\\128\\162 Couldn't match expected type
 \\226\\128\\152Int\\226\\128\\153 with actual type
 \\226\\128\\152()\\226\\128\\153\n\
                          \    \\226\\128\\162 In the second argument of
 \\226\\128\\152(<>)\\226\\128\\153, namely
 \\226\\128\\152b\\226\\128\\153\n\
                          \      In the expression: a <> b\n\
                          \      In an equation for
 \\226\\128\\152x\\226\\128\\153: x b = a <> b\n\
                          \(deferred type error)"#
 }}}

 Here's my ~/.ghci config:

 {{{
 :set -fdefer-type-errors
 :set -XOverloadedStrings
 :set +t
 }}}

 Both -fdefer-type-errors and -XOVerloadedStrings are necessary to cause
 the panic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13510>
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