[FIX] web: proper rendering of widget tag

There was no consideration of the fact that the widget
rendering process could be asynchronous. This commit
fixes that situation so that render a node of type
<widget name="..."> is now possible.

addons/web/static/src/js/views/basic/basic_renderer.js

@@ -629,15 +629,23 @@ var BasicRenderer = AbstractRenderer.extend({
 
         // Prepare widget rendering and save the related deferred
         var def = widget._widgetRenderAndInsert(function () {});
-        if (def.state() === 'pending') {
+        var async = def.state() === 'pending';
+        if (async) {
             this.defs.push(def);
         }
+        var $el = async ? $('<div>') : widget.$el;
 
-        // handle other attributes/modifiers
-        this._handleAttributes(widget.$el, node);
-        this._registerModifiers(node, record, widget);
-        widget.$el.addClass('o_widget');
-        return widget.$el;
+        var self = this;
+        def.then(function () {
+            self._handleAttributes(widget.$el, node);
+            self._registerModifiers(node, record, widget);
+            widget.$el.addClass('o_widget');
+            if (async) {
+                $el.replaceWith(widget.$el);
+            }
+        });
+
+        return $el;
     },
 
     /**

addons/web/static/tests/views/form_tests.js

@@ -7172,6 +7172,35 @@ QUnit.module('Views', {
         assert.ok("Behavior can't be tested");
     });
 
+    QUnit.test('asynchronous rendering of a widget tag', function (assert) {
+        assert.expect(1);
+
+        var def1 = $.Deferred();
+
+        var MyWidget = Widget.extend({
+            willStart: function() {
+                return def1;
+            },
+        });
+
+        widgetRegistry.add('test', MyWidget);
+
+        createAsyncView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form>' +
+                        '<widget name="test"/>' +
+                    '</form>',
+        }).then(function(form) {
+            assert.strictEqual(form.$('div.o_widget').length, 1,
+                "there should be a div with widget class");
+            form.destroy();
+            delete widgetRegistry.map.test;
+        });
+
+        def1.resolve();
+    });
 });
 
 });