[GHC] #10447: DeriveFoldable rejects instances with constraints in last argument of data type

[GHC]

#10447: DeriveFoldable rejects instances with constraints in last argument of data
type
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        Reporter:  RyanGlScott       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #8678             |  Differential Revisions:
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Comment (by RyanGlScott):

 I'm probably making an assumption about how robust GHC's equality
 constraint solver is, but I would think that it'd be capable enough to
 infer whether an argument's type is equal to the type parameter. If it
 were undecidable, then wouldn't this (legal) code have to be rejected?

 {{{#!hs
 data S a b where
     MkS :: (a ~ Int) => Int -> S a b

 deriving instance Foldable (S a)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10447#comment:9>
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