Fix #13797: .is with single-node context

(cherry picked from commit 4f786ba4)

src/traversing.js

@@ -58,14 +58,16 @@ jQuery.fn.extend({
 	},
 
 	is: function( selector ) {
-		return !!selector && (
-			typeof selector === "string" ?
-				// If this is a positional/relative selector, check membership in the returned set
-				// so $("p:first").is("p:last") won't return true for a doc with two "p".
-				rneedsContext.test( selector ) ?
-					jQuery( selector, this.context ).index( this[ 0 ] ) >= 0 :
-					jQuery.filter( selector, this ).length > 0 :
-				this.filter( selector ).length > 0 );
+		return !!winnow(
+			this,
+
+			// If this is a positional/relative selector, check membership in the returned set
+			// so $("p:first").is("p:last") won't return true for a doc with two "p".
+			typeof selector === "string" && rneedsContext.test( selector ) ?
+				jQuery( selector ) :
+				selector || [],
+			false
+		).length;
 	},
 
 	closest: function( selectors, context ) {

test/unit/traversing.js

@@ -141,16 +141,22 @@ test("is() with :has() selectors", function() {
 });
 
 test("is() with positional selectors", function() {
-	expect(24);
-
-	var isit = function(sel, match, expect) {
-		equal( jQuery( sel ).is( match ), expect, "jQuery('" + sel + "').is('" + match + "')" );
-	};
-
-	jQuery(
-		"<p id='posp'><a class='firsta' href='#'><em>first</em></a><a class='seconda' href='#'><b>test</b></a><em></em></p>"
-	).appendTo( "#qunit-fixture" );
+	expect(27);
+
+	var
+		posp = jQuery(
+			"<p id='posp'><a class='firsta' href='#'><em>first</em></a>" +
+			"<a class='seconda' href='#'><b>test</b></a><em></em></p>"
+		).appendTo( "#qunit-fixture" ),
+		isit = function( sel, match, expect ) {
+			equal(
+				jQuery( sel ).is( match ),
+				expect,
+				"jQuery('" + sel + "').is('" + match + "')"
+			);
+		};
 
+	isit( "#posp", "p:last", true );
 	isit( "#posp", "#posp:first", true );
 	isit( "#posp", "#posp:eq(2)", false );
 	isit( "#posp", "#posp a:first", false );
@@ -179,6 +185,9 @@ test("is() with positional selectors", function() {
 	isit( "#posp em", "#posp a em:eq(2)", false );
 
 	ok( jQuery("#option1b").is("#select1 option:not(:first)"), "POS inside of :not() (#10970)" );
+
+	ok( jQuery( posp[0] ).is("p:last"), "context constructed from a single node (#13797)" );
+	ok( !jQuery( posp[0] ).find("#firsta").is("a:first"), "context derived from a single node (#13797)" );
 });
 
 test("index()", function() {