[GHC] #12239: Dependent type family does not reduce

[GHC]

#12239: Dependent type family does not reduce
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
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 int-index):

 > Does this sound like something you might tackle?


 Yes. Your explanation is very helpful.


 > I'm sure bumps will come up along the road, but if you're feeling like
 getting your hands dirty here, I can continue to advise.


 Thank you. So, my understanding is that during type checking `ForAllTy`s
 arise, but the `Kind`s in `TyVarBinder`s don't get flattened, hence the
 error. However, once I added a `traceFlat` to the beginning of
 `flatten_one` clause for `ForAllTy` it didn't show up in the compilation
 logs (adding it in other clauses worked, so I assume that I added it
 correctly). Therefore I conclude that control flow doesn't reach this part
 of the code and no changes will affect the result of compilation.

 (1) What other parts of code should be changed and how, before I can start
 fixing `flatten_one` for `ForAllTy`?

 (2) Where exactly those `ForAllTy` get inserted in the example code (in
 the ticket description)?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12239#comment:8>
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