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

[GHC]

#14845: TypeInType, index GADT by constraint witness
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:  fixed             |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  dependent/should_fail/T14845_fail1,
                                     |  dependent/should_fail/T14845_fail2
      Blocked By:                    |             Blocking:
 Related Tickets:  #13895, #15215,   |  Differential Rev(s):  Phab:D4728
  #15282                             |
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  patch => closed
 * testcase:   => dependent/should_fail/T14845_fail1,
     dependent/should_fail/T14845_fail2
 * resolution:   => fixed
 * related:  #13895, #15215 => #13895, #15215, #15282
 * milestone:   => 8.6.1


Comment:

 Commit c63754118cf6c3d0947d0c611f1db39c78acf1b7 improves the error message
 substantially.

 As mentioned in comment:4, we can't actually allow this program until
 dependent types are available.

 The remaining task from this ticket is to document how a program like the
 one in comment:16 desugars into coercions at the Core Level. I've opened
 #15282 for this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14845#comment:21>
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