[GHC] #13324: Allow PartialTypeSignatures in the instance context of a standalone deriving declaration

[GHC]

#13324: Allow PartialTypeSignatures in the instance context of a standalone
deriving declaration
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10607            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Since it's morally equivalent to just using a deriving Eq clause.

 And it'll be implementationally the same as using `deriving Eq` too!   So
 the same things will happen automatically.

 In the code, the `mtheta` variable will tbe `Nothing` rather than `Just`.

 Just allowing a wildcard on its own would be a good start.  But we have
 code for doing the "subtraction" in cases like `(Eq a, _) => blah`, so it
 would not be too hard to support that too.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13324#comment:2>
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