[GHC] #15009: Float equalities past local equalities

[GHC]

#15009: Float equalities past local equalities
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  gadt/T15009
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nfrisby):

 For future interested parties (e.g. me :/), I suggest reading these Notes
 in this order. (These links are into the tip of the {{{ghc-8.6}}} branch
 as of this moment.)

   1. {{{Note [TcLevel and untouchable type variables]}}}, {{{Note
 [WantedInv]}}}, {{{Note [Skolem escape prevention]}}}, and {{{Note
 [TcLevel assignment]}}} in the {{{Untoucable type variables}}} [sic]
 section starting at
 [https://github.com/ghc/ghc/blob/8bed140099f8ab78e3e728fd2e50dd73d7210e84/compiler/typecheck/TcType.hs#L679
 TcType.hs line 679]
   1. {{{Note [Float Equalities out of Implications]}}}, {{{Note [Float
 equalities from under a skolem binding]}}}, {{{Note [Which equalities to
 float]}}}, {{{Note [Skolem escape]}}}, and {{{Note [What prevents a
 constraint from floating]}}} within the {{{Floating equalities}}} section
 starting at
 [https://github.com/ghc/ghc/blob/8bed140099f8ab78e3e728fd2e50dd73d7210e84/compiler/typecheck/TcSimplify.hs#L2132
 TcSimplify.hsline 2132]
   1. {{{Note [When does an implication have given equalities?]}}} and
 {{{Note [Let-bound skolems]}}} starting at
 [https://github.com/ghc/ghc/blob/8bed140099f8ab78e3e728fd2e50dd73d7210e84/compiler/typecheck/TcSMonad.hs#L2142
 TcSMonad.hs line 2142]

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15009#comment:15>
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