[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:  patch
        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):  Phab:D4728
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D4728


Comment:

 Phab:D4728 attempts to provide a better error message when promoting a
 data constructor with an unpromotable context.

 Note that I didn't attempt to change `checkValidType` in this patch, since
 that would require a significant amount of plumbing to check whether
 `checkValidType` was being called on a type or a kind. I felt that the
 existing error message from `tcInstBinder` (which I tweaked slightly in
 this Diff to say "kind" instead of "type") was sufficient.

 This patch also doesn't check for type synonyms, as suggested in
 comment:15, since that would require changing the checks in `tcInstBinder`
 accordingly, and I don't know how to do that. But in any case, this is a
 solid improvement over the status quo.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14845#comment:17>
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