The constructor invoked may be either another auxiliary constructor or the primary constructor, and it must be the first statement in the auxiliary constructor's body.
Traits don't support auxiliary constructors, nor do they accept an argument list for the primary constructor, the body of a trait.
Traits不支持辅助构造函数，它们也不支持在主构造函数，trait的主体里的参数列表。
3
Because all auxiliary constructors eventually invoke the primary constructor, logic checks and other initializations done in the body will be performed consistently for all instances created.