[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 simonpj):

 > I might be operating under a misconception here, but isn't it impossible
 to properly detect wildcards in a type

 What do you mean by "properly detect"? How would things be better if we
 did "properly detect" them?  Specifically

 * Why will the error messages get better? Can you give an example?
 * Is that an argument for putting wildcards everywhere? Why should only
 instance declarations get the benefit of this error message improvement?
 * Why couldn't we just improve the error messages for `LHsSigType`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:10>
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