[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):

 Well by making it strict you'll certainly force the `tyCoVarsOfTypes` to
 happen, which isn't really a good idea.

 But there is something weird here: why is `exprToType` being called on an
 expression other than `(Type ty)`.  Let's find out.  You can make the
 panic print the expr. And it could come from
 {{{
 applyTypeToArg fun_ty arg = piResultTy fun_ty (exprToType arg)
 }}}
 perhpas?  Which call to `exprToType` (there aren't many) is at fault?

 More and more mysterious

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371#comment:38>
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