[GHC] #13885: Template Haskell doesn't freshen GADT type variables properly

[GHC]

#13885: Template Haskell doesn't freshen GADT type variables properly
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Why does this matter?  If I write
 {{{
 data T a b where
   MkT :: [a] -> b -> x -> T x a
 }}}
 that's absolutely fine.  The 'a' and 'b' in the `MkT` signature have
 nothing to do with the variables in the head.  The latter do not scope
 over the constructors.  Right?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13885#comment:2>
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