Custom option keyword :set-after.

etc/NEWS

@@ -385,6 +385,21 @@ cause the customizations to fail in earlier versions of Emacs.
 Custom buffers when you've done with them or just bury them (the
 default).
 
+*** The keyword :set-after in defcustom allows to specify dependencies
+between custom options.  Example:
+
+  (defcustom default-input-method nil
+    "*Default input method for multilingual text (a string).
+  This is the input method activated automatically by the command
+  `toggle-input-method' (\\[toggle-input-method])."
+    :group 'mule
+    :type '(choice (const nil) string)
+    :set-after '(current-language-environment))
+
+This specifies that default-input-method should be set after
+current-language-environment even if default-input-method appears
+first in a custom-set-variables statement.
+
 ** New features in evaluation commands
 
 The commands to evaluate Lisp expressions, such as C-M-x in Lisp