[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 dolio):

 I'm not asking if GHC would derive the instance I specified. I'm asking
 whether it's an okay `Foldable` instance based on the same reasoning as
 for:

 {{{
 data Bar a where
   Bar :: (a ~ Int) -> Int -> Bar a
 }}}

 and the like. "Can we automatically do this," is not the only relevant
 question. "Should we do this and why," also matters.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10447#comment:12>
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