[GHC] #15147: Type checker plugin receives Wanteds that are not completely unflattened

[GHC]

#15147: Type checker plugin receives Wanteds that are not completely unflattened
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        Reporter:  nfrisby           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeCheckerPlugins
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 adamgundry):

 Replying to [comment:11 simonpj]:
 > > The issue here is that the wanteds are now sometimes not fully
 unflattened, isn't it?
 >
 > No, the reported issue (in the Description) is that a Given `CFunEqCan`
 remains.  I believe that all Wanted `CFunEqCans` are removed.

 Perhaps I'm misunderstanding something, but as the Description shows, the
 plugin is being called with
 {{{
  [WD] hole{co_a4or} {3}:: (F x_a4o6[tau:2] fsk_a4oc[fsk:0] :: *)
                            ~# (p_a4o2[sk:2] :: *) (CNonCanonical)
 }}}
 including the flatten-skolem `fsk_a4oc`. Shouldn't that be unflattened to
 give
 {{{
  (F x_a4o6[tau:2] (G () ()) :: *) ~# (p_a4o2[sk:2] :: *)
 }}}
 aka `(p ~ F x (G () ()))`?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15147#comment:12>
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