[GHC] #11699: Type families mistakingly report kind variables as unbound type variables

[GHC]

#11699: Type families mistakingly report kind variables as unbound type variables
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           Reporter:  mniip          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
           Keywords:                 |  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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 GHC verifies that if some equation of a type family binds a type variable,
 that this type variable actually exists and doesn't disappear through type
 synonym/family application (#7536). However this also mistakingly catches
 kind variables that aren't present in the type family head.

 Simplest test case:
 {{{#!hs
 {-# LANGUAGE TypeFamilies, PolyKinds #-}
 type family F a where F (f a) = f a
 }}}
 As seen on 8.0.1-rc2 and 7.10.2:
 {{{
 ../File.hs:3:23:
     Family instance purports to bind type variable ‘k1’
       but the real LHS (expanding synonyms) is: F (f a) = ...
     In the equations for closed type family ‘F’
     In the type family declaration for ‘F’
 }}}

 The culprit seems to be in `exactTyCoVarsOfType`, which doesn't grab kind
 variables from a type variable's kind, even though `tyCoVarsOfType` does.

 Now, I'm not too sure whether this is a "GHC rejects valid program", or
 "Incorrect warning at compile time", as I'm not sure if type families like
 the aforementioned `F` are actually okay. (Are they?)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11699>
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