[GHC] #9032: Panic with self-import

[GHC]

#9032: Panic with self-import
-----------------------------------+---------------------------------------
       Reporter:  jstolarek        |             Owner:
           Type:  bug              |            Status:  new
       Priority:  normal           |         Milestone:
      Component:  Compiler         |           Version:  7.8.2
       Keywords:                   |  Operating System:  Unknown/Multiple
   Architecture:                   |   Type of failure:  Compile-time crash
  Unknown/Multiple                 |         Test Case:
     Difficulty:  Unknown          |          Blocking:
     Blocked By:                   |
Related Tickets:                   |
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 I've run into GHC panic with this code:

 {{{
 {-# OPTIONS_GHC -fno-warn-unused-imports #-}

 module Singletons.Star where

 import Data.Singletons.Prelude
 import Data.Singletons.Decide
 import Data.Singletons.CustomStar
 import Singletons.Nat
 import Singletons.Star  -- <------- HERE

 data Vec :: * -> Nat -> * where
   VNil :: Vec a Zero
   VCons :: a -> Vec a n -> Vec a (Succ n)

 $(singletonStar [''Nat, ''Int, ''String, ''Maybe, ''Vec])
 }}}

 This is a test in `singletons` package that leads to a panic when run:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.8.2 for x86_64-unknown-linux):
         tcIfaceGlobal (local): not found:
     singletons-1.0:Singletons.Star.TFCo:R:DemoteRep*KProxy{tc r0}
     []

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 I was unable to minimize the test case. Reproducing requires following
 steps:

 1. Installing latest th-desugar library from github:

 {{{
 git clone https://github.com/goldfirere/th-desugar.git
 cd th-desugar
 cabal install
 }}}

 2. Getting latest development version of singletons:

 {{{
 git clone https://github.com/goldfirere/singletons
 cd singletons
 git checkout 56734c2bda721cb9a6a3021b901e3b29d6564f0c
 make tests
 }}}

 All should go well. Now you need to edit file `tests/compile-and-
 dump/Singletons/Star.hs` and uncomment `import Singletons.Star` line. It
 should be possible to reproduce the bug with:

 {{{
 cd tests/compile-and-dump/ghc -package-name singletons-1.0
 Singletons/Star.hs -i../../dist/build -c -XTemplateHaskell
 }}}

 It's important to run `make tests` first so that all interface files
 required by `Star.hs` are in place.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9032>
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