[GHC] #11361: Test tc253 doesn't pass with reversed uniques

[GHC]

#11361: Test tc253 doesn't pass with reversed uniques
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        Reporter:  niteria           |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by niteria):

 I believe the problem is here:

 https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/typecheck/TcTyClsDecls.hs;c78fedde7055490ca6f6210ada797190f3c35d87$1003

 There's no reason to believe that `ktvs` and `mkTyVarTys fam_tc_tvs` will
 be in the matching order, and indeed `ktvs` is in the order of uniques.

 We start out with:

 {{{
 type Fam a_a18o3H b_a18o3G :: *
 type Fam a_a18o3F x_a18o3E = FamHelper a_a18o3F x_a18o3E
 }}}

 then when we reach `tcDefaultAssocDecl` we have:

 `FamHelper (a_a18o3F |> <*>_N) (x_a18o3E |> <*>_N) |> <*>_N)`

 `ktvs` is `[x_a18o3E, a_a18o3F]`

 `mkTyVarTys fam_tc_tvs` is `[a_a18o3H, b_a18o3G]`

 When we zip them together we end up with a definition that flips the
 arguments.
 From there it goes downhill, because when we try to compute `Fam Int Bool`
 we do it with flipped arguments: `FamHelper Bool Int` which is `(String,
 Bool)` which eventually doesn't unify with `Maybe Int`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11361#comment:4>
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