Include missing optional properties in contextually typed object literal

src/compiler/checker.ts

@@ -5455,6 +5455,16 @@ module ts {
                 }
             }
 
+            // If object literal is contextually typed, add missing optional properties from the contextual type
+            // such that the resulting type becomes a subtype in cases where only optional properties were omitted
+            if (contextualType) {
+                forEach(getPropertiesOfObjectType(contextualType), p => {
+                    if (p.flags & SymbolFlags.Optional && !hasProperty(properties, p.name)) {
+                        properties[p.name] = p;
+                    }
+                });
+            }
+
             var stringIndexType = getIndexType(IndexKind.String);
             var numberIndexType = getIndexType(IndexKind.Number);
             var result = createAnonymousType(node.symbol, properties, emptyArray, emptyArray, stringIndexType, numberIndexType);