[GHC] #12785: GHC panic, TypeFamily in equality constraint

[GHC]

#12785: GHC panic, TypeFamily in equality constraint
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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 heisenbug):

 With the attached patch I get:
 {{{
 $ ghci Tree.hs
 GHCi, version 8.1.20161030: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Tree             ( Tree.hs, interpreted )

 Tree.hs:29:63: error:
     • Could not deduce: Payload ('S n1) (Payload n1 s1) ~ s
         arising from a use of ‘SBranchX’
       from the context: m1 ~ 'S n1
         bound by a pattern with constructor:
                    Branch :: forall a (n :: Peano).
                              a -> HTree n (HTree ('S n) a) -> HTree ('S n)
 a,
                  in an equation for ‘nest’
         at Tree.hs:29:7-51
     • In the second argument of ‘($)’, namely ‘a `SBranchX` tr’
       In the expression: Hide $ a `SBranchX` tr
       In an equation for ‘nest’:
           nest (Hide a `Branch` (nest . hmap nest -> Hide tr))
             = Hide $ a `SBranchX` tr
     • Relevant bindings include
         tr :: STree n1 (STree ('S n1) (STree ('S ('S n1)) f)) s1
           (bound at Tree.hs:29:49)
         a :: STree ('S m1) f s (bound at Tree.hs:29:12)
         nest :: HTree m1 (Hidden ('S m1) f) -> Hidden m1 (STree ('S m1) f)
           (bound at Tree.hs:27:1)
 Failed, modules loaded: none.
 }}}

 Please review... (and consider the note above)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:3>
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