[GHC] #12159: Record-like GADTs with repeated fields (of same type) rejected

[GHC]

#12159: Record-like GADTs with repeated fields (of same type) rejected
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        Reporter:  heisenbug         |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by adamgundry):

 A few of us discussed this and believe that both cases above can be
 covered with a single generalisation: for all the constructors that
 mention the field, require that the field types are the same, then take
 the anti-unifier of all the result types and check that this includes all
 the variables mentioned by the common field type. For example:

 {{{#!hs
 data S x where
   S1 :: { f :: Maybe a } -> S ([a], Int)
   S2 :: { f :: Maybe a } -> S ([a], Bool)

 f :: S ([a], b) -> Maybe a
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12159#comment:5>
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