[GHC] #12966: GHC panics with forall and constraint

[GHC]

#12966: GHC panics with forall and constraint
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        Reporter:  xcmw              |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  wontfix           |             Keywords:
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 bgamari):

 * status:  new => closed
 * resolution:   => wontfix
 * milestone:   => 8.2.1


Comment:

 Thanks for the report, xcmw!

 On `master` this expectedly fails with,

 {{{
 λ> :set -XRankNTypes
 λ> type Maybeify c = forall d. (c d) => ((~) (Maybe d))

 <interactive>:3:39: error:
     • Expecting one more argument to ‘(~) (Maybe d)’
       Expected a type, but ‘(~) (Maybe d)’ has kind ‘* -> Constraint’
     • In the type ‘forall d. (c d) => ((~) (Maybe d))’
       In the type declaration for ‘Maybeify’
 }}}

 This was likely fixed by #11660, which won't be backported to 8.0.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12966#comment:1>
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