While typing up ticket:13761#comment:137502, I came across a poor error message around CUSKs.

data Proxy (a :: k) = P
data S :: forall k. Proxy k -> Type where
  MkS :: S (P :: Proxy Maybe)

produces

    You have written a *complete user-suppled kind signature*,
    but the following variable is undetermined: k0 :: *
    Perhaps add a kind signature.
    Inferred kinds of user-written variables:

That promised list of the kinds of user-written variables is empty. Either GHC should find something to print (like k :: k0, perhaps) or omit the header.