Scala 模式匹配守卫打破了模式匹配的穷尽性？

Question

为什么在 Scala 中模式匹配中断模式匹配穷尽性时保护一个案例？

chris@chris-870Z5E-880Z5E-680Z5E:~/dev/bitcoins-core$ sbt console
[info] Loading project definition from /home/chris/dev/bitcoins-core/project
[info] Set current project to bitcoin-s-core (in build file:/home/chris/dev/bitcoins-core/)
[info] Starting scala interpreter...
[info] 
Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_101).
Type in expressions to have them evaluated.
Type :help for more information.

scala> sealed trait A 
defined trait A

scala> case class B() extends A 
defined class B

scala> case class C() extends A 
defined class C

scala> val b : A = B() 
b: A = B()

scala> b match { case b : B => () }
<console>:15: warning: match may not be exhaustive.
It would fail on the following input: C()
       b match { case b : B => () }
       ^

scala> b match { case b : B if b.isInstanceOf[B] => ()  }

scala> 

请注意最后一行没有详尽警告，我是不是漏掉了什么？

Answer 1

看起来这是一个 open issue .

A comment by Adriaan Moors :

I understand it's disappointing, and it's not very intuitive why we bail out when there are guards, but our key design goal is not to have spurious warnings, as that would undermine their utility while annoying a lot of users.