Deferred: Make jQuery.when synchronous when possible

Fixes jquerygh-3100

src/deferred.js

@@ -366,16 +366,19 @@ 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 ( resolveValues[ i ] && jQuery.isFunction( 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

@@ -1049,3 +1049,24 @@ 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() {
+			assert.equal( when, "before" );
+		},
+		done = assert.async( 5 );
+
+	jQuery.when().done( validate ).always( done );
+	jQuery.when( when ).done( validate ).always( done );
+	jQuery.when( resolved ).done( validate ).always( done );
+	jQuery.when( rejected ).fail( validate ).always( done );
+	jQuery.when( resolved, rejected ).always( validate ).always( done );
+
+	when = "after";
+} );