Force `brew upgrade` to treat arguments as casks if `--cask` is passed.

Library/Homebrew/cli/named_args.rb

@@ -79,8 +79,8 @@ module Homebrew
                                   .freeze
       end
 
-      def to_resolved_formulae_to_casks
-        @to_resolved_formulae_to_casks ||= to_formulae_and_casks(method: :resolve)
+      def to_resolved_formulae_to_casks(only: nil)
+        @to_resolved_formulae_to_casks ||= to_formulae_and_casks(method: :resolve, only: only)
                                            .partition { |o| o.is_a?(Formula) }
                                            .map(&:freeze).freeze
       end

Library/Homebrew/cmd/upgrade.rb

@@ -91,7 +91,10 @@ module Homebrew
   def upgrade
     args = upgrade_args.parse
 
-    formulae, casks = args.named.to_resolved_formulae_to_casks
+    only = :formula if args.formula? && !args.cask?
+    only = :cask if args.cask? && !args.formula?
+
+    formulae, casks = args.named.to_resolved_formulae_to_casks(only: only)
     # If one or more formulae are specified, but no casks were
     # specified, we want to make note of that so we don't
     # try to upgrade all outdated casks.