[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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Changes (by simonpj):

 * cc: core-libraries-committee@… (added)


Comment:

 Copying the Core Libraries Committee.  What conditions ''are'' ok?  For
 example, is this OK?
 {{{
 data T a where
   MkT :: (a~Int) => a -> T a
 }}}
 or this (which is equivalent)
 {{{
 data T a where
   MkT :: Int -> T Int
 }}}
 I think that the 'deriving' code for `Functor`, `Foldable` etc behaves
 specially when it comes across the universally-quantified type variable
 (`a` above), so it might get confused if it was `Int` and not `a`.

 Disabling the test is easy: the question is whether that's the right thing
 to do.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10447#comment:2>
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