index now works in reverse when passed a selector or undefined. fixes #3971

src/core.js

@@ -182,10 +182,16 @@ jQuery.fn = jQuery.prototype = {
 	// Determine the position of an element within
 	// the matched set of elements
 	index: function( elem ) {
+		if ( !elem || typeof elem === "string" ) {
+			return jQuery.inArray( this[0],
+				// If it receives a string, the selector is used
+				// If it receives nothing, the siblings are used
+				elem ? jQuery( elem ) : this.parent().children() );
+		}
 		// Locate the position of the desired element
 		return jQuery.inArray(
 			// If it receives a jQuery object, the first element is used
-			elem && elem.jquery ? elem[0] : elem, this );
+			elem.jquery ? elem[0] : elem, this );
 	},
 
 	is: function( selector ) {

test/unit/core.js

@@ -366,12 +366,13 @@ test("each(Function)", function() {
 	ok( pass, "Execute a function, Relative" );
 });
 
-test("index(Object)", function() {
-	expect(10);
+test("index(Object|String|undefined)", function() {
+	expect(15);
 
 	var elements = jQuery([window, document]),
 		inputElements = jQuery('#radio1,#radio2,#check1,#check2');
 
+	// Passing a node
 	equals( elements.index(window), 0, "Check for index of elements" );
 	equals( elements.index(document), 1, "Check for index of elements" );
 	equals( inputElements.index(document.getElementById('radio1')), 0, "Check for index of elements" );
@@ -381,9 +382,18 @@ test("index(Object)", function() {
 	equals( inputElements.index(window), -1, "Check for not found index" );
 	equals( inputElements.index(document), -1, "Check for not found index" );
 
+	// Passing a jQuery object
 	// enabled since [5500]
 	equals( elements.index( elements ), 0, "Pass in a jQuery object" );
 	equals( elements.index( elements.eq(1) ), 1, "Pass in a jQuery object" );
+	equals( jQuery("#form :radio").index( jQuery("#radio2") ), 1, "Pass in a jQuery object" );
+
+	// Passing a selector or nothing
+	// enabled since [6329]
+	equals( jQuery('#text2').index(), 2, "Check for index amongst siblings" );
+	equals( jQuery('#form').children().eq(4).index(), 4, "Check for index amongst siblings" );
+	equals( jQuery('#radio2').index('#form :radio') , 1, "Check for index within a selector" );
+	equals( jQuery('#radio2').index('#form :text') , -1, "Check for index not found within a selector" );
 });
 
 test("jQuery.merge()", function() {