Suppose we've written an application library in Unison, We could do some test to make sure that it works as expected.

在Unison中使用google test时，发现EXPECT_EQ在fail时，不能打印Unison Test Language中定义的派生类的对象。于是写了个纯C++的示例，发现在只定义基类的operator<<时，无法打印派生类对象。