patch 7.4.786

Problem:    It is not possible for a plugin to adjust to a changed setting.
Solution:   Add the OptionSet autocommand event. (Christian Brabandt)

runtime/doc/autocmd.txt

@@ -259,6 +259,7 @@ Name			triggered by ~
 |Syntax|		when the 'syntax' option has been set
 |EncodingChanged|	after the 'encoding' option has been changed
 |TermChanged|		after the value of 'term' has changed
+|OptionSet|		after setting any option
 
 	Startup and exit
 |VimEnter|		after doing all the startup stuff
@@ -744,6 +745,27 @@ MenuPopup			Just before showing the popup menu (under the
 					o	Operator-pending
 					i	Insert
 					c	Command line
+							*OptionSet*
+OptionSet			After setting an option.  The pattern is
+				matched against the long option name.
+				The |v:option_old| variable indicates the
+				old option value, |v:option_new| variable
+				indicates the newly set value, the
+				|v:option_type| variable indicates whether
+				it's global or local scoped and |<amatch>|
+				indicates what option has been set.
+
+				Is not triggered on startup and for the 'key'
+				option for obvious reasons.
+
+				Note: It's a bad idea, to reset an option
+				during this autocommand, since this will
+				probably break plugins. You can always use
+				|noa| to prevent triggering this autocommand.
+				Could be used, to check for existence of the
+				'backupdir' and 'undodir' options and create
+				directories, if they don't exist yet.
+
 							*QuickFixCmdPre*
 QuickFixCmdPre			Before a quickfix command is run (|:make|,
 				|:lmake|, |:grep|, |:lgrep|, |:grepadd|,

runtime/doc/eval.txt

@@ -1535,6 +1535,15 @@ v:oldfiles	List of file names that is loaded from the |viminfo| file on
 		than String this will cause trouble.
 		{only when compiled with the |+viminfo| feature}
 
+						    *v:option_new*
+v:option_new    New value of the option. Valid while executing an |OptionSet|
+		autocommand.
+						    *v:option_old*
+v:option_old    Old value of the option. Valid while executing an |OptionSet|
+		autocommand.
+						    *v:option_type*
+v:option_type   Scope of the set command. Valid while executing an
+		|OptionSet| autocommand. Can be either "global" or "local"
 					*v:operator* *operator-variable*
 v:operator	The last operator given in Normal mode.  This is a single
 		character except for commands starting with <g> or <z>,