a: A 是类型归属；参见 Scala 中类型归属的目的是什么?

: _* 是类型归属的特殊实例，它告诉编译器将序列类型的单个参数视为可变参数序列，即 varargs.

: _* is a special instance of type ascription which tells the compiler to treat a single argument of a sequence type as a variable argument sequence, i.e. varargs.

使用 Queue.apply 创建一个 Queue 是完全有效的，该Queue.apply 具有单个元素是序列或可迭代的，所以这正是当你给单个 Iterable[A].

It is completely valid to create a Queue using Queue.apply that has a single element which is a sequence or iterable, so this is exactly what happens when you give a single Iterable[A].