[GHC] #11371: Bogus in-scope set in substitutions

[GHC]

#11371: Bogus in-scope set in substitutions
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11360            |  Differential Rev(s):  phab:D1792,
       Wiki Page:                    |  phab:D1801, phab:D1802
-------------------------------------+-------------------------------------

Comment (by thomie):

 This commit causes the following Travis failures with DEBUG=YES:
 {{{
 Unexpected failures:
    ../../libraries/base/tests/IO  hClose002 [exit code non-0] (normal)
    ../../libraries/base/tests/IO  hClose003 [exit code non-0] (normal)
    ../../libraries/base/tests/IO  hDuplicateTo001 [exit code non-0]
 (normal)
 }}}

 See https://s3.amazonaws.com/archive.travis-ci.org/jobs/105899161/log.txt

 Here is one of them:
 {{{
 =====> hClose002(normal) 4736 of 5007 [0, 0, 0]
 Compile failed (status 256) errors were:
 [1 of 1] Compiling Main             ( hClose002.hs, hClose002.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20160130 for x86_64-unknown-linux):
         ASSERT failed!
   CallStack (from ImplicitParams):
   assertPprPanic, called at compiler/types/TyCoRep.hs:1903:56 in
 ghc:TyCoRep
   checkValidSubst, called at compiler/types/TyCoRep.hs:2047:17 in
 ghc:TyCoRep
   substCo, called at compiler/basicTypes/MkId.hs:698:28 in ghc:MkId
   in_scope InScope {}
   tenv [a1oW :-> dev_a1Z8, a1oX :-> enc_state_a1Z9,
         a1oY :-> dec_state_a1Za]
   tenvFVs [a1Z8 :-> dev_a1Z8, a1Z9 :-> enc_state_a1Z9,
            a1Za :-> dec_state_a1Za]
   cenv []
   cenvFVs []
   tys []
   cos [N:IORef[0] <Buffer Word8>_N]
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371#comment:24>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler