[GHC] #10361: DeriveAnyClass does not fill in associated type defaults

28 Apr 2015

      #10361: DeriveAnyClass does not fill in associated type defaults
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              Reporter:  kosmikus    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 I would expect `test1` and `test2` below to typecheck.

 This is a reduced test case from trying to use `DeriveAnyClass` on the
 `Generic` class of the `generics-sop` package, which unfortunately fails
 due to this bug.

 {{{
 {-# LANGUAGE DeriveAnyClass, StandaloneDeriving, TypeFamilies #-}
 module Test where

 class C1 a where
   type T1 a
   type instance T1 a = Char

 class C2 a where -- equivalent to C1
   type T2 a
   type instance T2 a = Char

 class C3 a where -- equivalent to C1, C2
   type T3 a
   type instance T3 a = Char

 data A = B
   deriving C1

 deriving instance C2 A

 instance C3 A

 -- fails
 -- test1 :: T1 A
 -- test1 = 'x'

 -- fails
 -- test2 :: T2 A
 -- test2 = 'x'

 -- succeeds
 test3 :: T3 A
 test3 = 'x'
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10361
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