Deferred: Make jQuery.when synchronous when possible

Closes gh-3102
Fixes gh-3100
Closes gh-3105

src/deferred.js

@@ -366,16 +366,21 @@ jQuery.extend( {
 
 		// Single- and empty arguments are adopted like Promise.resolve
 		if ( remaining <= 1 ) {
-			adoptValue( singleValue, master.resolve, master.reject );
+			adoptValue( singleValue, master.done( updateFunc( i ) ).resolve, master.reject );
 
 			// Use .then() to unwrap secondary thenables (cf. gh-3000)
-			return master.then();
+			if ( master.state() === "pending" ||
+				jQuery.isFunction( resolveValues[ i ] && resolveValues[ i ].then ) ) {
+
+				return master.then();
+			}
 		}
 
 		// Multiple arguments are aggregated like Promise.all array elements
 		while ( i-- ) {
 			adoptValue( resolveValues[ i ], updateFunc( i ), master.reject );
 		}
+
 		return master.promise();
 	}
 } );

test/unit/deferred.js

@@ -824,24 +824,27 @@ QUnit.test( "jQuery.when(nonThenable) - like Promise.resolve", function( assert
 QUnit.test( "jQuery.when(thenable) - like Promise.resolve", function( assert ) {
 	"use strict";
 
-	assert.expect( 56 );
+	var CASES = 16,
-
+		slice = [].slice,
-	var slice = [].slice,
 		sentinel = { context: "explicit" },
 		eventuallyFulfilled = jQuery.Deferred().notify( true ),
 		eventuallyRejected = jQuery.Deferred().notify( true ),
+		secondaryFulfilled = jQuery.Deferred().resolve( eventuallyFulfilled ),
+		secondaryRejected = jQuery.Deferred().resolve( eventuallyRejected ),
 		inputs = {
 			promise: Promise.resolve( true ),
 			rejectedPromise: Promise.reject( false ),
 			deferred: jQuery.Deferred().resolve( true ),
 			eventuallyFulfilled: eventuallyFulfilled,
-			secondaryFulfilled: jQuery.Deferred().resolve( eventuallyFulfilled ),
+			secondaryFulfilled: secondaryFulfilled,
+			eventuallySecondaryFulfilled: jQuery.Deferred().notify( true ),
 			multiDeferred: jQuery.Deferred().resolve( "foo", "bar" ),
 			deferredWith: jQuery.Deferred().resolveWith( sentinel, [ true ] ),
 			multiDeferredWith: jQuery.Deferred().resolveWith( sentinel, [ "foo", "bar" ] ),
 			rejectedDeferred: jQuery.Deferred().reject( false ),
 			eventuallyRejected: eventuallyRejected,
-			secondaryRejected: jQuery.Deferred().resolve( eventuallyRejected ),
+			secondaryRejected: secondaryRejected,
+			eventuallySecondaryRejected: jQuery.Deferred().notify( true ),
 			multiRejectedDeferred: jQuery.Deferred().reject( "baz", "quux" ),
 			rejectedDeferredWith: jQuery.Deferred().rejectWith( sentinel, [ false ] ),
 			multiRejectedDeferredWith: jQuery.Deferred().rejectWith( sentinel, [ "baz", "quux" ] )
@@ -857,6 +860,7 @@ QUnit.test( "jQuery.when(thenable) - like Promise.resolve", function( assert ) {
 			deferred: [ true ],
 			eventuallyFulfilled: [ true ],
 			secondaryFulfilled: [ true ],
+			eventuallySecondaryFulfilled: [ true ],
 			multiDeferred: [ "foo", "bar" ],
 			deferredWith: [ true ],
 			multiDeferredWith: [ "foo", "bar" ]
@@ -866,6 +870,7 @@ QUnit.test( "jQuery.when(thenable) - like Promise.resolve", function( assert ) {
 			rejectedDeferred: [ false ],
 			eventuallyRejected: [ false ],
 			secondaryRejected: [ false ],
+			eventuallySecondaryRejected: [ false ],
 			multiRejectedDeferred: [ "baz", "quux" ],
 			rejectedDeferredWith: [ false ],
 			multiRejectedDeferredWith: [ "baz", "quux" ]
@@ -875,7 +880,9 @@ QUnit.test( "jQuery.when(thenable) - like Promise.resolve", function( assert ) {
 		// Strict mode functions invoked without .call/.apply get global-object context
 		defaultContext = (function getDefaultContext() { return this; }).call(),
 
-		done = assert.async( 28 );
+		done = assert.async( CASES * 2 );
+
+	assert.expect( CASES * 4 );
 
 	jQuery.each( inputs, function( message, value ) {
 		var code = "jQuery.when( " + message + " )",
@@ -917,6 +924,8 @@ QUnit.test( "jQuery.when(thenable) - like Promise.resolve", function( assert ) {
 	setTimeout( function() {
 		eventuallyFulfilled.resolve( true );
 		eventuallyRejected.reject( false );
+		inputs.eventuallySecondaryFulfilled.resolve( secondaryFulfilled );
+		inputs.eventuallySecondaryRejected.resolve( secondaryRejected );
 	}, 50 );
 } );
 
@@ -1049,3 +1058,28 @@ QUnit.test( "jQuery.when - notify does not affect resolved", function( assert )
 		assert.ok( false, "Error on resolve" );
 	} );
 } );
+
+QUnit.test( "jQuery.when(...) - opportunistically synchronous", function( assert ) {
+
+	assert.expect( 5 );
+
+	var when = "before",
+		resolved = jQuery.Deferred().resolve( true ),
+		rejected = jQuery.Deferred().reject( false ),
+		validate = function( label ) {
+			return function() {
+				assert.equal( when, "before", label );
+			};
+		},
+		done = assert.async( 5 );
+
+	jQuery.when().done( validate( "jQuery.when()" ) ).always( done );
+	jQuery.when( when ).done( validate( "jQuery.when(nonThenable)" ) ).always( done );
+	jQuery.when( resolved ).done( validate( "jQuery.when(alreadyFulfilled)" ) ).always( done );
+	jQuery.when( rejected ).fail( validate( "jQuery.when(alreadyRejected)" ) ).always( done );
+	jQuery.when( resolved, rejected )
+		.always( validate( "jQuery.when(alreadyFulfilled, alreadyRejected)" ) )
+		.always( done );
+
+	when = "after";
+} );