[GHC] #14584: Core Lint error

[GHC]

#14584: Core Lint error
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  DeferredTypeErrors
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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"f5cf9d1a1b198edc929e1fa96c6d841d182fe766/ghc"
 f5cf9d1a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f5cf9d1a1b198edc929e1fa96c6d841d182fe766"
 Fix floating of equalities

 This rather subtle patch fixes Trac #14584.  The problem was
 that we'd allowed a coercion, bound in a nested scope, to escape
 into an outer scope.

 The main changes are

 * TcSimplify.floatEqualities takes more care when floating
   equalities to make sure we don't float one out that mentions
   a locally-bound coercion.
   See Note [What prevents a constraint from floating]

 * TcSimplify.emitResidualConstraints (which emits the residual
   constraints in simplifyInfer) now avoids burying the constraints
   for escaping CoVars inside the implication constraint.

 * Since I had do to this stuff with CoVars, I moved the
   fancy footwork about not quantifying over CoVars from
   TcMType.quantifyTyVars to its caller
   TcSimplify.decideQuantifiedTyVars.  I think its other
   callers don't need to worry about all this CoVar stuff.

 This turned out to be surprisigly tricky, and took me a solid
 day to get right.  I think the result is reasonably neat, though,
 and well documented with Notes.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14584#comment:10>
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