[GHC] #14845: TypeInType, index GADT by constraint witness

[GHC]

#14845: TypeInType, index GADT by constraint witness
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13895            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #13895


Comment:

 I too have hit this limitation—or rather, I have hit this error message—in
 #13895. But in the case of #13895, the root of the issue is
 impredicativity, whereas here I'm not sure that's the case.

 I'd be curious to hear goldfire's reasoning for why this restriction is in
 place, since the
 [http://git.haskell.org/ghc.git/blob/34834234fff4a9dd0408d3b29e001cd132665327:/compiler/typecheck/Inst.hs#l451
 source code] doesn't have much in the way of an explanation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14845#comment:2>
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