[GHC] #13415: Instance declarations don't recognize named wildcards

[GHC]

#13415: Instance declarations don't recognize named wildcards
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13324            |  Differential Rev(s):  Phab:D3332
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The point that I'm trying to make is that we're gradually transitioning
 instance declarations over to recognizing wildcard types, so this is a
 necessary first step to take in that direction. Having recognized this, we
 need to `Wc`-ify the `LHsSigType`s used in instance declaration AST types.
 Having done so, we will get the //immediate// benefit of having better
 error messages when named wildcards are used, and we will get the knock-on
 benefit of making it easier for named wildcard to be properly integrated
 later.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:7>
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