[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):

 OK so there are two separate things

 * Error messages.  You say "_x is a named wildcard and GHC is not
 detecting it".  So perhaps, with `NamedWildCards` we should complain about
 `_x` in any `LHsSigType`, rather than treating it as an ordinary type
 variable.  Good plan: that would nail the error message issue.

 * Extending instance declarations to support wildcards.  You want to do
 this in two steps. I'd prefer to take the second step (getting the payoff)
 before committing the first.  Step 2 may turn out to influence Step 1.
 (E.g. the only wildcard we want to allow is in the context.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:12>
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