Deferred: syncronize single and multiple target handling in $.when

Fixes gh-2546
Fixes gh-2018
Close gh-2707

src/deferred.js

@@ -294,19 +294,17 @@ jQuery.extend( {
 	},
 
 	// Deferred helper
-	when: function( subordinate /* , ..., subordinateN */ ) {
+	when: function() {
 		var method,
 			i = 0,
 			resolveValues = slice.call( arguments ),
 			length = resolveValues.length,
 
 			// the count of uncompleted subordinates
-			remaining = length !== 1 ||
-				( subordinate && jQuery.isFunction( subordinate.promise ) ) ? length : 0,
+			remaining = length,
 
 			// the master Deferred.
-			// If resolveValues consist of only a single Deferred, just use that.
-			master = remaining === 1 ? subordinate : jQuery.Deferred(),
+			master = jQuery.Deferred(),
 
 			// Update function for both resolve and progress values
 			updateFunc = function( i, contexts, values ) {
@@ -316,14 +314,17 @@ jQuery.extend( {
 					if ( values === progressValues ) {
 						master.notifyWith( contexts, values );
 					} else if ( !( --remaining ) ) {
-						master.resolveWith( contexts, values );
+						master.resolveWith(
+							contexts.length === 1 ? contexts[ 0 ] : contexts,
+							values
+						);
 					}
 				};
 			},
 			progressValues, progressContexts, resolveContexts;
 
 		// Add listeners to Deferred subordinates; treat others as resolved
-		if ( length > 1 ) {
+		if ( length > 0 ) {
 			progressValues = new Array( length );
 			progressContexts = new Array( length );
 			resolveContexts = new Array( length );
@@ -345,14 +346,13 @@ jQuery.extend( {
 						updateFunc( i, progressContexts, progressValues )
 					);
 				} else {
-					--remaining;
-				}
+					updateFunc( i, resolveContexts, resolveValues )( resolveValues[ i ] );
 				}
 			}
 
 		// If we're not waiting on anything, resolve the master
-		if ( !remaining ) {
-			master.resolveWith( resolveContexts, resolveValues );
+		} else {
+			master.resolveWith();
 		}
 
 		return master.promise();

test/unit/deferred.js

@@ -667,7 +667,6 @@ QUnit.test( "jQuery.when", function( assert ) {
 		"undefined": undefined,
 		"a plain object": {},
 		"an array": [ 1, 2, 3 ]
-
 	}, function( message, value ) {
 		assert.ok(
 			jQuery.isFunction(
@@ -698,12 +697,10 @@ QUnit.test( "jQuery.when", function( assert ) {
 	} );
 
 	jQuery.each( [ 1, 2, 3 ], function( k, i ) {
-
 		jQuery.when( cache || jQuery.Deferred( function() {
 				this.resolve( i );
 			} )
 		).done( function( value ) {
-
 			assert.strictEqual( value, 1, "Function executed" + ( i > 1 ? " only once" : "" ) );
 			cache = value;
 		} );
@@ -759,10 +756,8 @@ QUnit.test( "jQuery.when - joined", function( assert ) {
 				expected = shouldResolve ? [ 1, 1 ] : [ 0, undefined ],
 				expectedNotify = shouldNotify && [ willNotify[ id1 ], willNotify[ id2 ] ],
 				code = "jQuery.when( " + id1 + ", " + id2 + " )",
-				context1 = defer1 && jQuery.isFunction( defer1.promise ) ? defer1.promise() :
-					( defer1.then ? window : undefined ),
-				context2 = defer2 && jQuery.isFunction( defer2.promise ) ? defer2.promise() :
-					( defer2.then ? window : undefined );
+				context1 = defer1 && jQuery.isFunction( defer1.promise ) ? defer1.promise() : window,
+				context2 = defer2 && jQuery.isFunction( defer2.promise ) ? defer2.promise() : window;
 
 			jQuery.when( defer1, defer2 ).done( function( a, b ) {
 				if ( shouldResolve ) {
@@ -880,3 +875,36 @@ QUnit.test( "jQuery.when - chaining", function( assert ) {
 
 	defer.resolve( "other deferred" );
 } );
+
+QUnit.test( "jQuery.when - solitary thenables", function( assert ) {
+
+	assert.expect( 1 );
+
+	var done = assert.async(),
+		rejected = new Promise( function( resolve, reject ) {
+			setTimeout( function() {
+				reject( "rejected" );
+			}, 100 );
+		} );
+
+	jQuery.when( rejected ).then(
+		function() {
+			assert.ok( false, "Rejected, solitary, non-Deferred thenable should not resolve" );
+			done();
+		},
+		function() {
+			assert.ok( true, "Rejected, solitary, non-Deferred thenable rejected properly" );
+			done();
+		}
+	);
+} );
+
+QUnit.test( "jQuery.when does not reuse a solitary jQuery Deferred (gh-2018)", function( assert ) {
+
+	assert.expect( 2 );
+	var defer = jQuery.Deferred().resolve(),
+		promise = jQuery.when( defer );
+
+	assert.equal( promise.state(), "resolved", "Master Deferred is immediately resolved" );
+	assert.notStrictEqual( defer.promise(), promise, "jQuery.when returns the master deferred's promise" );
+} );