[GHC] #13848: Unexpected order of variable quantification with GADT constructor

[GHC]

#13848: Unexpected order of variable quantification with GADT constructor
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeApplications
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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Comment (by RyanGlScott):

 I don't think it's going to be that simple. If you look at the
 implementation of `mkDataConRep`, you'll notice that it figures out the
 type of the `wrap_id` like this:

 {{{#!hs
 wrap_ty = dataConUserType data_con
 }}}

 So obviously we don't want to change `dataConUserType`'s implementation to
 use `idType wrap_id`, since that would be a circular definition.

 > We should ensure that the wrapper type reflects exactly the type the
 user wrote including type variable order

 Sure, but that's easier said than done. The only info you have in
 `mkDataConId` to determine the wrapper type is the `DataCon` itself, and
 at that point, the type variables have already been carved up into the
 universal and existential ones, with no way to recover the original order.
 So I don't see any way to make this work without having a separate
 `dcOrigTyVars` fields, as proposed in comment:2.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13848#comment:4>
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