[GHC] #8263: allow duplicate deriving / standalone deriving

[GHC]

#8263: allow duplicate deriving / standalone deriving
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        Reporter:  aavogt            |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:  wontfix           |             Keywords:  deriving
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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Changes (by monoidal):

 * status:  new => closed
 * resolution:   => wontfix


Comment:

 In the specific case of Typeable, since GHC 8.2 every data type is an
 instance of Typeable and we're ignoring "deriving instance Typeable"
 altogether.

 In general case, there's the problem of deriving strategies from
 [comment:3 comment:3]. We could, in principle, allow the declaration
 "deriving S instance T" if T was derived using the same strategy S
 elsewhere. However, this only solves the issue if the T was derived; if
 there's an "instance" declaration, I don't see any nice way to do it. So
 this feature would be rather limited in power; we don't have examples
 other than Typeable, and in the meantime #8100 was fixed.

 I'm closing the ticket; please reopen if you still need this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8263#comment:5>
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