[GHC] #4896: Deriving Data does not work for attached code

[GHC]

#4896: Deriving Data does not work for attached code
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              Reporter:  mitar       |            Owner:
                  Type:  bug         |           Status:  infoneeded
              Priority:  low         |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.1
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by thomie):

 * status:  new => infoneeded


Comment:

 This is the error message with HEAD (7.9.20141125) for the example from
 the description.
 {{{
 Main.hs:17:66:
     Couldn't match type ‘t'0 a0’ with ‘Bar’
     Expected type: Maybe (c (Bar (D a b)))
       Actual type: Maybe (c (t'0 a0 (D a b)))
     In the expression: gcast2 f
     In an equation for ‘dataCast2’: dataCast2 f = gcast2 f
     When typechecking the code for  ‘dataCast2’
       in a derived instance for ‘Data (Bar (D a b))’:
       To see the code I am typechecking, use -ddump-deriv
 }}}

 And the error message for mojojojo's example from comment:6 with HEAD (see
 also #5863):
 {{{
 test.hs:5:40:
     Deriving Typeable is not allowed for family instances;
     derive Typeable for ‘DF’ alone
     In the data instance declaration for ‘DF’

 test.hs:6:42:
     Deriving Typeable is not allowed for family instances;
     derive Typeable for ‘DF’ alone
     In the data instance declaration for ‘DF’
 }}}

 Is there still a problem here?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4896#comment:8>
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