[GHC] #12936: Type inference regression in GHC HEAD

[GHC]

#12936: Type inference regression in GHC HEAD
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"f1036ad80efb9cf80977fa234f8b9c7b23cc6835/ghc" f1036ad/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f1036ad80efb9cf80977fa234f8b9c7b23cc6835"
 Make dropDerivedSimples restore [WD] constraints

 I'd forgotten to turn [W] + [D] constraints back into [WD]
 in dropDerivedSimples; and that led to Trac #12936.

 Fortunately the fix is simple.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12936#comment:4>
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