[GHC] #9757: Warn about derivable instances

[GHC]

#9757: Warn about derivable instances
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  closed
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:  wontfix     |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 I'm not sure if this works with the way HLint is organized, but there is a
 decent way of detecting placing where GND can be used: take the Core
 produced from the instance definition of a newtype `N` (wrapping
 representation type `T`), strip out all coercions, and compare it against
 the Core for `T`'s instance. If each method in the instance for `N` is
 either a call to the method for `T`'s instance ''or'' symbol-for-symbol
 identical to `T`s instance, then GND is suitable.

 I think this algorithm would catch the common cases easily enough. But,
 like I said, I don't know how well looking at Core code matches with the
 way HLint normally goes about its business.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9757#comment:8>
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