[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):

 What would you like it to say?  You get the exact same message from
 {{{
 instance x => Show (Maybe a)
 }}}
 namely
 {{{
 Foo.hs:4:10: error:
     * Illegal constraint: x (Use ConstraintKinds to permit this)
     * In the context: x
       While checking an instance declaration
       In the instance declaration for `Show (Maybe a)'
   |
 4 | instance x => Show (Maybe a)
   |          ^^^^^^^^^^^^^^^^^^^
 }}}
 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.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:4>
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