Instance declarations don't recognize named wildcards

While exploring a fix for #13324 (closed), I was blocked pretty quickly by this limitation: instance declarations don't properly recognize named wildcards. Here is an example to demonstrate:

{-# 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.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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