不一致的null相等性检查scala 2.11.7 [英] Inconsistent null equality check scala 2.11.7

问题描述

Scala 2.12.6中不再存在此问题

This issue no longer exists in Scala 2.12.6

原始问题(针对Scala 2.11.7):

Original question (for Scala 2.11.7):

为什么这么奇怪的警告?

Why so strange warning?

scala> null.asInstanceOf[Double]
res0: Double = 0.0

scala> null.asInstanceOf[Double] == null
<console>:11: warning: comparing values of types 
      Double and Null using `==' will always yield !!!!false!!!!
       null.asInstanceOf[Double] == null
                                 ^
res1: Boolean = true //!!!!

scala> 0.0 == null
<console>:11: warning: comparing values of types Double and Null using `==' will always yield false
       0.0 == null
           ^
res2: Boolean = false

scala> null.asInstanceOf[Double] == 0.0
res6: Boolean = true

scala> val a = null.asInstanceOf[Double]
a: Double = 0.0

scala> a == null
<console>:12: warning: comparing values of types Double and Null using `==' will always yield false
       a == null
         ^
res7: Boolean = false

P.S. Int和Long

P.S.2这不是重复的-这里的问题是，无论asInstanceOf(如您从我的回答中看到的那样)，根本不会发生装箱+警告消息不一致

P.S.2 It's not a duplicate - the problem here is that boxing doesn't happen at all regardless of asInstanceOf (as you can see from my answer) + the warning message is inconsistent

推荐答案

此 issue 在Scala 2.12.6中不再存在.请参见拉动请求及其说明.

This issue no longer exists in Scala 2.12.6. See pull-request with explanation.

原始答案(针对Scala 2.11.7):

Original answer (for Scala 2.11.7):

null.asInstanceOf[Double] == null编译为:

aconst_null
ifnonnull

val版本可编译为:

aconst_null
invokestatic unboxToDouble
putfield
aload_0
invokevirtual а
invokestatic boxToDouble
ifnonnull

所以编译器只是忘了在第一种情况下添加unbox/box

So compiler just forgets to add unbox/box in the first case

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