[GHC] #15592: Type families without CUSKs cannot be given visible kind variable binders

[GHC]

#15592: Type families without CUSKs cannot be given visible kind variable binders
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  TypeApplications, TypeFamilies,
                                     |  CUSKs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  14880             |             Blocking:
 Related Tickets:  #15591            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:10 simonpj]:
 > Hmm.  I wonder if we should simply add `a` to the `tyConScopedTyVars` of
 `T`?  And that info ''is'' available in `kcLHsTQTVars` which builds the
 `TcTyCon`.

 I too am a bit confused as to why we don't add `a` (or any kind variables,
 for that matter) to `tcTyConScopedTyVars` in the non-CUSK case of
 `kcLHsQTyVars`. The only reasoning I could find for this design choice is
 [http://git.haskell.org/ghc.git/blob/fc2ff6dd7496a33bf68165b28f37f40b7d647418:/compiler/typecheck/TcHsType.hs#l1609
 this terse comment]:

 {{{#!hs
        ; let   -- NB: Don't add scoped_kvs to tyConTyVars, because they
                -- must remain lined up with the binders
              tc_binders = zipWith mk_tc_binder hs_tvs tc_tvs
              tycon = mkTcTyCon name (ppr user_tyvars) tc_binders res_kind
                                (mkTyVarNamePairs (scoped_kvs ++ tc_tvs))
                                flav
 }}}

 (Here, they call `tcTyConScopedTyVars` "`tyConTyVars`".)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15592#comment:11>
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