Revert 8d1c422

src/core.js

@@ -422,11 +422,8 @@ jQuery.extend({
 	},
 
 	isPlainObject: function( obj ) {
-		var key;
 		// Not plain objects:
 		// - Any object or value whose internal [[Class]] property is not "[object Object]"
-		//   ...Unless it was a constructor, whose prototype property was paved over by
-		//   by a plain object at its declaration. #13571
 		// - DOM nodes
 		// - window
 		if ( jQuery.type( obj ) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
@@ -445,11 +442,9 @@ jQuery.extend({
 			return false;
 		}
 
-		// Own properties are enumerated firstly, so to speed up,
-		// if last one is own, then all properties are own.
-		for ( key in obj ) {}
-
-		return key === undefined || core_hasOwn.call( obj, key );
+		// If the function hasn't returned already, we're confident that
+		// |obj| is a plain object, created by {} or constructed with new Object
+		return true;
 	},
 
 	isEmptyObject: function( obj ) {

test/unit/core.js

@@ -296,9 +296,9 @@ test("type", function() {
 });
 
 asyncTest("isPlainObject", function() {
-	expect(16);
+	expect(15);
 
-	var pass, iframe, doc, c, extended,
+	var pass, iframe, doc,
 		fn = function() {};
 
 	// The use case that we want to match
@@ -360,45 +360,6 @@ asyncTest("isPlainObject", function() {
 	} catch(e) {
 		window.iframeDone( Object, "iframes not supported" );
 	}
-
-	// #13571
-	function C() {}
-	C.prototype = {
-		x: 1
-	};
-	c = new C();
-
-	extended = jQuery.extend( true, {}, {
-		target: c
-	});
-
-	strictEqual(
-		extended.target, c,
-		"Instances, whose constructor defined its prototype by assigning a plain object, " +
-		"will lie about their true identity to preserve a broken user-code expectation"
-	);
-	//
-	// The test above is broken and tests a broken feature, to support a misinformed
-	// assumption, as documented here:
-	//
-	// http://bugs.jquery.com/ticket/13571#comment:4
-	//
-	// It will not pass if the object being assigned as the prototype
-	// has no properties:
-	//
-	// function C() {}
-	// C.prototype = {};
-	// c = new C();
-
-	// extended = jQuery.extend( true, {}, {
-	// 	target: c
-	// });
-
-	// strictEqual( extended.target, c, "Undetectable, will fail every time" );
-	//
-	// The solution is to reset the constructor property of your plain object prototypes.
-	//
-	//
 });
 
 test("isFunction", function() {