[GHC] #13738: TypeApplications-related GHC internal error

[GHC]

#13738: TypeApplications-related GHC internal error
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13985            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Ugh. Trying to change anything involving `bindLHsTyVarBndr` is proving to
 be a massive headache. There are numerous complications here that I'm not
 sure how to deal with:

 * Using `rnImplicitBinders` simply won't work in `bindHsQTyVars`.
 `rnImplicitBinders` expects an `LHsType GhcPs` as an argument, which
 `bindHsQTyVars` doesn't have.
 * Even if we did somehow pass an `LHsType GhcPs` to `rnImplicitBndrs` from
 `bindHsQTyVars`, I'm not convinced it would do the right thing. The
 problem: the `kv_bndrs` from `bindHsQTyVars` need to be bound using
 `newTyVarNameRn mb_assoc` (since the `kv_bndrs` might be from an
 associated class), but `rnImplicitBndrs` does nothing of the sort.
 * `bindLHsTyVarBndr` performs a validity check
 ([http://git.haskell.org/ghc.git/blob/7463a95dbe53d789f8f245f26735a7ac74bb6e11:/compiler/rename/RnTypes.hs#l974
 here]) to ensure that a list of tyvar binders is well scoped (i.e., reject
 something like `forall (a :: k) k.`). But we can only perform this check
 because `bindLHsTyVarBndrs` keeps track of the kind variables that are
 implicitly bound so far, one by one, and communicates this information to
 each call to `bindLHsTyVarBndr` so that it can check if a binder is in the
 set of implicitly bound kind variables. If we removed `bindLHsTyVarBndr`'s
 ability to implicitly bind kind variables, I'm not sure how we'd perform
 this check at all.
 * Could we move this error check to `rnImplicitBndrs`? Perhaps, but then
 we'd need to migrate the code which tracks one-by-one all of the
 implicitly bound kind variables so far. That doesn't sound fun.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13738#comment:13>
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