[GHC] #14748: Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int))

31 Jan 2018

      #14748: Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int))
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints wipT2893     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 (based on WIP [https://phabricator.haskell.org/D4353
 -XQuantifiedConstraints])

 Could GHC infer the context for deriving `Data (Foo f)` (#12245)?

 {{{#!hs
 data Foo f = Foo (f Bool) (f Int)
   deriving stock Data

 {-
 Generates:

 instance (Typeable f, (forall xx. Data xx => Data (f xx))) => Data (Foo f)
 -}
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14748
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