# 4.4.4-9 Sum 类型

Sum 类型用来表示一个类型集合，其定义如下：

```go
type Sum struct {
	types []Type // types are unique
}
```

Sum 当前用在`Interface`类型的 types 属性中，用于表示泛型的限定类型列表（constraint list）。该类型也是编译器内部定义的辅助类型结构，不是语言层面的类型。


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