[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:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 I came across a curious bug with record-like GADTs and repeated fields.
 Consider following code:

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 data Foo p where
   Bar :: { quux' :: Bool } -> Foo Char
   Baz :: { quux'' :: Bool } -> Foo Int

 quux :: Foo p -> Bool
 quux (Bar q) = q
 quux (Baz q) = q

 quuxSetter :: Foo p -> Bool -> Foo p
 quuxSetter old at Bar{} q = old{quux' = q}
 quuxSetter old at Baz{} q = old{quux'' = q}
 }}}

 This compiles and all is fine. *But* GHC is supposed to create the nice
 `quux` and `quuxSetter` accessors for me, right?

 So, let's try:
 {{{#!hs
 data Foo p where
   Bar :: { quux :: Bool } -> Foo Char
   Baz :: { quux :: Bool } -> Foo Int
 }}}
 It does not compile! Instead I get:
 {{{
 T12159.hs:3:1: error:
       Constructors Bar and Baz have a common field  quux ,
         but have different result types
       In the data type declaration for  Foo
 Failed, modules loaded: none.
 }}}
 This is not very polite :-) It should simply create the accessors like I
 did above. It obviously can be done!

 Testcase is attached.

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