Deprecate brew cask update.

Library/Homebrew/cask/lib/hbc/cli.rb

@@ -19,7 +19,7 @@ require "hbc/cli/reinstall"
 require "hbc/cli/search"
 require "hbc/cli/style"
 require "hbc/cli/uninstall"
-require "hbc/cli/update"
+require "compat/hbc/cli/update"
 require "hbc/cli/zap"
 
 require "hbc/cli/internal_use_base"

Library/Homebrew/cask/lib/hbc/cli/update.rb

@@ -1,20 +0,0 @@
-module Hbc
-  class CLI
-    class Update < Base
-      def self.run(*_ignored)
-        result = SystemCommand.run(HOMEBREW_BREW_FILE,
-                                   args: ["update"])
-        # TODO: separating stderr/stdout is undesirable here.
-        #       Hbc::SystemCommand should have an option for plain
-        #       unbuffered output.
-        print result.stdout
-        $stderr.print result.stderr
-        exit result.exit_status
-      end
-
-      def self.help
-        "a synonym for 'brew update'"
-      end
-    end
-  end
-end

Library/Homebrew/compat/hbc/cli/update.rb

@@ -0,0 +1,17 @@
+module Hbc
+  class CLI
+    class Update < Base
+      def self.run(*_ignored)
+        odeprecated "`brew cask update`", "`brew update`", disable_on: Time.utc(2017, 7, 1)
+        result = SystemCommand.run(HOMEBREW_BREW_FILE, args:         ["update"],
+                                                       print_stderr: true,
+                                                       print_stdout: true)
+        exit result.exit_status
+      end
+
+      def self.help
+        "a synonym for 'brew update'"
+      end
+    end
+  end
+end