GHCi's :kind doesn't work with wildcards

Witness this GHCi session:

rae:20:35:37 ~/ghc/ghc-head/ghc> ~/ghc/ghc-head/inplace/bin/ghc-stage2 --interactive
GHCi, version 8.7.20180716: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /Users/rae/.ghc/ghci.conf
Prelude> :set -XPartialTypeSignatures
Prelude> import Data.Proxy
Prelude Data.Proxy> :k Proxy _
Proxy _ :: *
Prelude Data.Proxy> :k Proxy (Maybe :: _)

<interactive>:1:8: error:
    • Expecting one more argument to ‘Maybe’
      Expected kind ‘_’, but ‘Maybe’ has kind ‘* -> *’
    • In the first argument of ‘Proxy’, namely ‘(Maybe :: _)’
      In the type ‘Proxy (Maybe :: _)’

It seems we're not doing the correct wildcard thing here: GHCi should report that _ should be Type -> Type.

I believe that this is because TcHsType.tcWildCardBinders uses newSkolemTyVar internally; tcWildCardBinders is called from TcRnDriver.tcRnType, which is the implementation of GHCi's :kind. This is GHC's only call of tcWildCardBinders. All other places use tcWildCardBindersX newWildTyVar, which makes vastly more sense.

I would just fix this myself, but it smells intentional. Does anyone know why we have all this?

(Credit to @int-index who found this bug.)