[GHC] #15515: Bogus "No instance" error when type families appear in kinds

[GHC]

#15515: Bogus "No instance" error when type families appear in kinds
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 The class instance should be rejected -- indeed, I'm quite surprised it's
 not.

 Let's write with explicit kinds:

 {{{#!hs
 class C k a where ...
 instance C (F -> Type) D where ...
 }}}

 That instance mentions a type family in one of its arguments, which should
 be rejected.

 I would love to come up with a way where we ignore determined dependent
 arguments during matching, which would then allow this instance to work,
 but we're not there yet.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15515#comment:1>
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