[GHC] #12120: GHC accepts invalid Haskell: `class Eq (a Int) => C a where`

[GHC]

#12120: GHC accepts invalid Haskell: `class Eq (a Int) => C a where`
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        Reporter:  thomie            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  report-impact
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by thomie):

 I didn't see this mentioned in https://downloads.haskell.org/~ghc/master
 /users-guide/bugs.html. It probably should be, so that the Haskell 2020
 commitee sees it, and perhaps lifts this restriction on class and instance
 declarations.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12120#comment:1>
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