[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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Comment (by RyanGlScott):

 But there's a significant wrinkle here. You're proposing that we change
 the error message to something like `Data constructor has a constraint
 'cls0 a0', and thus cannot be promoted`, yes? But the place where this
 error message is thrown is `tcInstBinders`, which is not in any way data-
 constructor–specific. Indeed, as I mentioned in comment:2, this error
 message also occurs in the program in #13895, but that doesn't promote any
 data constructors whatsoever.

 So if we change this error message _carte blanche_, all of a sudden GHC is
 going to be complaining about data constructors which don't exist.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14845#comment:10>
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