[GHC] #15591: Inconsistent kind variable binder visibility between associated and non-associated type families

[GHC]

#15591: Inconsistent kind variable binder visibility between associated and non-
associated type families
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  TypeApplications, TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Oops, it just hit me why I was getting `forall {k}. Maybe k` for `ZT`'s
 kind even after the patch in comment:1 — `ZT` doesn't have a CUSK! (See
 #15592 for more on this point in particular.)

 If I change the definition to this:

 {{{#!hs
 class Z (k :: Type) where
   type ZT :: Maybe k
   type ZT = (Nothing :: Maybe k)
 }}}

 //Then// I get `forall k. Maybe k` for the kind of `ZT`, as expected, and
 the default definition is no longer rejected. Yay!

 But all is not well yet. There remains the mystery of why the CUSK-less
 version of `ZT` is being rejected after applying the patch in comment:1.
 One clue is that if I apply the patch in comment:1 (and comment out the
 default definition for `ZT`), then GHCi gives me the following kind for
 `Z`:

 {{{
 Z :: forall {k}. k -> Constraint
 }}}

 This doesn't seem right, since without the patch, I get the following kind
 for `Z`:

 {{{
 Z :: * -> Constraint
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15591#comment:3>
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