Using GHC head at diff Visible kind application D5229 (#12045 (closed)),

bug goes away if you remove @ob.

{-# Language RankNTypes #-}
{-# Language TypeApplications #-}
{-# Language DataKinds        #-}
{-# Language PolyKinds        #-}
{-# Language GADTs            #-}
{-# Language TypeFamilies     #-}

import Data.Kind

class Ríki (ob :: Type) where
 type Hom :: ob -> ob -> Type

data
  Kl_kind :: forall ob . (ob -> ob) -> ob -> Type where
  Kl      :: k -> Kl_kind @ob (m) k

type family
  UnKl (kl :: Kl_kind m k) = (res :: k) where
  UnKl (Kl a) = a

$ ghci -ignore-dot-ghci 543_bug.hs
GHCi, version 8.7.20181017: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( 543_bug.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20181017 for x86_64-unknown-linux):
        ASSERT failed!
  Type-correct unfilled coercion hole {co_a1yH}
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable
        assertPprPanic, called at compiler/typecheck/TcHsSyn.hs:1805:99 in ghc:TcHsSyn

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

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