[GHC] #9211: Untouchable type variable (regression from 7.6)

[GHC]

#9211: Untouchable type variable (regression from 7.6)
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        Reporter:  simonpj           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 This looks right to me, and 7.4 looks wrong.  There really is an equality
 `(f ~ g)` in the context, and so it's in general unsafe to fix `b` to
 `Bool`.  That's what the !OutsideIn paper is all about.

 In this case the equality is trivial but presumably it's not trivial in
 your real example.

 For a trivial equality like this, perhaps GHC is over-conservative, but
 lifting that would require yet more special-case pleading.   And I'm not
 sure it'd fix your real example anyway.

 In short, I don't know how to help.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9211#comment:2>
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