[GHC] #11105: Put the generic-default type in the Class

[GHC]

#11105: Put the generic-default type in the Class
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"924f851700ca9ba7e0e7483b7ca4de62a4a74e15/ghc"
 924f8517/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="924f851700ca9ba7e0e7483b7ca4de62a4a74e15"
 Refactor default methods (Trac #11105)

 This patch does some signficant refactoring to the treatment
 of default methods in class declarations, and more generally
 to the type checking of type/class decls.

 Highlights:

 * When the class has a generic-default method, such as
      class C a where
        op :: a -> a -> Bool
        default op :: Ord a => a -> a -> a
   the ClassOpItem records the type of the generic-default,
   in this case the type (Ord a => a -> a -> a)

 * I killed off Class.DefMeth in favour of the very-similar
   BasicTypes.DefMethSpec.  However it turned out to be better
   to use a Maybe, thus
       Maybe (DefMethSpec Type)
   with Nothing meaning "no default method".

 * In TcTyClsDecls.tcTyClGroup, we used to accumulate a [TyThing],
   but I found a way to make it much simpler, accumulating only
   a [TyCon].  Much less wrapping and unwrapping.

 * On the way I also fixed Trac #10896 in a better way. Instead
   of killing off all ambiguity checks whenever there are any type
   errors (the fix in commit 8e8b9ed), I instead recover in
   TcTyClsDecls.checkValidTyCl.

 There was a lot of associated simplification all round
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11105#comment:1>
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