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

[GHC]

#13415: Instance declarations don't recognize named wildcards
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While exploring a fix for #13324, I was blocked pretty quickly by this
 limitation: instance declarations don't properly recognize named
 wildcards. Here is an example to demonstrate:

 {{{#!hs
 {-# LANGUAGE NamedWildCards #-}
 module Bug where

 instance _x => Show (Maybe a)
 }}}

 {{{
 Bug.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)
   |          ^^^^^^^^^^^^^^^^^^^^
 }}}

 GHC doesn't recognize that `_x` is just a type variable, not a named
 wildcard.

 I believe fixing this is just a matter of changing the ASTs for instance
 declarations to use `LHsSigWcType` instead of `LHsSigType`. Patch
 incoming.

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