[GHC] #11084: Some type families don't reduce with :kind!

[GHC]

#11084: Some type families don't reduce with :kind!
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Sadly, I'm unable to reproduce this without `singletons`. :(

 Observe:

 {{{
 > ghci
 GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
 Prelude> import Data.Promotion.Prelude   -- singletons-2.0.1
 Prelude Data.Promotion.Prelude> :kind! Lookup 'True '[ '(True, True) ]
 Lookup 'True '[ '(True, True) ] :: Maybe Bool
 = Data.Singletons.Prelude.List.Case_1628035408
     'True
     'True
     'True
     '[]
     (Data.Singletons.Apply (Data.Singletons.Apply (:==$) 'True) 'True)
 Prelude Data.Promotion.Prelude> :kind! Lookup 'True '[ '(True, True) ]
 Lookup 'True '[ '(True, True) ] :: Maybe Bool
 = JustSym1 'True
 }}}

 Note that the exact same command run twice yields different results. I
 tried peeking at `-ddump-tc-trace` but got nothing of use.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11084>
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