[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):

 Replying to [comment:4 simonpj]:
 > What would you like it to say?  You get the exact same message from
 > {{{
 > instance x => Show (Maybe a)
 > }}}

 That isn't using a named wildcard, so I would expect that to give an error
 message involving `ConstraintKinds`. I wouldn't expect it from a named
 wildcard `_x`, however.

 > In any case, making instance declarations have `LHsSIgWcType` sounds as
 if instnace are ''allowed''' to have wildcards, but they aren't, so that
 feels like the wrong solution.

 It's true that instance heads aren't allowed to have wildcards. But then
 again, there are many other types in which wildcards are allowed to appear
 syntactically (e.g., `data Foo _`) but are later rejected, so we're not
 breaking convention by doing this. Besides, I don't see any simpler
 solution.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:5>
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