若一个类型指定了[类型参数]，则此类型成为泛型类型。
type List[T any] struct {
next *List[T]
value T
}类型参数
类型参数的语法如下：
TypeParameters = "[" TypeParamList [ "," ] "]" .
TypeParamList = TypeParamDecl { "," TypeParamDecl } .
TypeParamDecl = IdentifierList TypeConstraint .其中 TypeConstraint 即为：
TypeConstraint
TypeElem = TypeTerm { "|" TypeTerm } .
TypeTerm = Type | UnderlyingType .
UnderlyingType = "~" Type .