[GHC] #12124: Ambiguous type variable: it's a red herring!

[GHC]

#12124: Ambiguous type variable: it's a red herring!
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        Reporter:  drb226            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The case in comment:1 is behaving correctly.  The type of `x` is
 generalised to `x :: forall a. a`.  So the info from two uses of `x` in
 `(x, f x)` do not communicate with each other.

 Even `-XMonoLocalBinds` does not solve this, because with this flag GHC
 still generalises local bindings that have no free variables.

 I still need to look at the Description; but comment:1 looks fine to me.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12124#comment:2>
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