[GHC] #14332: Deriving clauses can have forall types

[GHC]

#14332: Deriving clauses can have forall types
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:16 simonpj]:
 > In comment:14 I say that GHC is behaving right.

 Except that it's clearly not, as evidenced in comment:7. I don't see a way
 to fix that short of overhauling how `deriving` clauses are
 renamed/typechecked, which I don't feel like I'd be able to accomplish
 without some mentoring.

 > You seem to think I'm asking for something subtle in the
 implementationn, but I'm not.

 I firmly disagree here—the way GHC typechecks `deriving` clauses is ripe
 with subtlety! It's caused me endless amounts of headaches trying to fix
 bugs like #10524 and #10561 in the past, as just two data points.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14332#comment:17>
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