[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):

 OK, I think I'm starting to see the picture more clearly. However, some of
 the details are still hazy to me. In particular, `tcConDecl` is far from
 the only place that uses `buildDataCon` to construct a `DataCon`. However,
 as far as I can tell, `tcConDecl` is the //only// place that knows exactly
 what order the user wrote the constructor's type variables in.

 This is important because we might write a GADT to an interface file and
 read it back in
 [http://git.haskell.org/ghc.git/blob/78c80c250021ccb7a84afaabebe0d69f9b9372ee:/compiler/iface/MkIface.hs#l1544
 tyConToIfaceDecl], which also uses `buildDataCon`. However, we've lost the
 order of the type variables used in the wrapper type by that point, since
 `IfaceData` doesn't store the wrapper type!

 This suggests to me that we need to beef up `IfaceData` to accommodate
 this change. Should we be storing the wrapper type as a field of
 `IfaceData` as well? Or am I barking up the wrong tree?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13848#comment:6>
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