为什么类型会导致“带有动物可序列化的产品”而不是动物？

Question

假设我有

val flag = true

为什么会导致类型Product with Serializable with Animal而不只是类型Animal？

class Animal(name : String)
case class Herbivore(name : String) extends Animal(name)
case class Carnivore(name : String) extends Animal(name)

val cow = new Herbivore("cow")
val tiger = new Carnivore("tiger")

if (flag) cow else tiger // Why is type Product with Serializable with Animal? 

Answer 1

案例类自动扩展， Product with Serializable因此

class Animal(name : String)
case class Herbivore(name : String) extends Animal(name)
case class Carnivore(name : String) extends Animal(name)

实际上是

class Animal(name : String)
case class Herbivore(name : String) extends Animal(name) with Product with Serializable
case class Carnivore(name : String) extends Animal(name) with Product with Serializable

因此，if (flag) cow else tiger编译器可以推断出的最精确的表达式类型是Product with Serializable with Animal。如果我们从案例类更改为像这样的类

class Animal(name : String)
class Herbivore(name : String) extends Animal(name)
class Carnivore(name : String) extends Animal(name)

那么if-else表达式的推断类型确实是Animal。

按照@TravisBrown的建议，使ADT根目录Product with Serializable像这样扩展

abstract class Animal(name : String) extends Product with Serializable
case class Herbivore(name : String) extends Animal(name)
case class Carnivore(name : String) extends Animal(name)

也会使if-else表达式的推断类型也变为Animal。