你通常会把函数返回值赋给拥有更特定类型变量的时候遇到这样的错误。
You usually see this error when you assign a value returned from a function to a variable with a more specific type.
即使你自己遇到这样的变量时，你也要不由的问自己：“这是在哪里定义的？” ，然后迅速的使用Ctrl+F组合在源代码里寻找这个变量最初定义的位置。
When you come across one you may ask yourself, "where was this declared?" and quickly mash Ctrl+F in a mad fit to find the variable's original source.
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