GHC panic with -fdefer-type-errors

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.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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