否则，如果有符号整型可以表示无符号整型的值时，公共类型是有符号类型。
Otherwise, if the signed integer type can represent all of the values of the unsigned type, the common type is the signed type.
否则，如果有符号整型可以表示无符号整型的值时，公共类型是有符号类型。
Otherwise, if the signed integer type can represent all of the values of the unsigned type, the common type is the signed type.
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