[GHC] #15828: Type family equation foralls allow strange re-quantification of class-bound type variables

[GHC]

#15828: Type family equation foralls allow strange re-quantification of class-bound
type variables
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by goldfire):

 * cc: mayac (added)


Comment:

 I vote strongly for `C2`. `C1` should be rejected because the local `a`
 shadows the class-bound `a`, and so the associated type family equation
 doesn't match its template.

 You forgot to mention that we need to

  3. Fix the implementation.

 @mayac, do you want to see this (and any other oddities that pop up)
 through? Or would you prefer that others (perhaps me) fix any problems
 that arise?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15828#comment:1>
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