[GHC] #12797: Default Rules stop working when providing some constraints

[GHC]

#12797: Default Rules stop working when providing some constraints
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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:"13508bad4810d4fa8581afbcb4f41c97fe4c92e2/ghc"
 13508bad/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="13508bad4810d4fa8581afbcb4f41c97fe4c92e2"
 Fix Trac #12797: approximateWC

 This patch makes approximateWC a bit more gung-ho when called
 from the defaulting code.  See Note [ApproximateWC], item (1).
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12797#comment:4>
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