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

[GHC]

#11371: Bogus in-scope set in substitutions
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        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
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Comment (by simonpj):

 While I'm thinking about this, I'd like to get rid of `composeTCvSubst`.
 It is somewhat complex, but it is used in very simple ways, basically as a
 form of `extendTCvSubst`.

 For example this call (in `checkAxInstCo`) is just stupid
 {{{
         subst        = zipTvSubst tvs tys `composeTCvSubst`
                        zipCvSubst cvs co_args
 }}}

 There are very few calls altogether. I think it could easily be nuked.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371#comment:17>
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