In TcValidity.checkAmbiguity, skolemise kind vars that appear free in the kinds of type variables

This was shown up by Trac #9222.

compiler/typecheck/TcValidity.lhs

@@ -74,11 +74,16 @@ checkAmbiguity ctxt ty
 
   | otherwise
   = do { traceTc "Ambiguity check for" (ppr ty)
-       ; (subst, _tvs) <- tcInstSkolTyVars (varSetElems (tyVarsOfType ty))
+       ; let free_tkvs = varSetElemsKvsFirst (closeOverKinds (tyVarsOfType ty))
+       ; (subst, _tvs) <- tcInstSkolTyVars free_tkvs
        ; let ty' = substTy subst ty
-              -- The type might have free TyVars,
-              -- so we skolemise them as TcTyVars
+              -- The type might have free TyVars, esp when the ambiguity check
+              -- happens during a call to checkValidType,
+              -- so we skolemise them as TcTyVars.
               -- Tiresome; but the type inference engine expects TcTyVars
+              -- NB: The free tyvar might be (a::k), so k is also free
+              --     and we must skolemise it as well. Hence closeOverKinds.
+              --     (Trac #9222)
 
          -- Solve the constraints eagerly because an ambiguous type
          -- can cause a cascade of further errors.  Since the free