Scala考古学：短絡評価は名前渡しとして実装されているか？ ref: https://qiita.com/kmizu/items/0925ab2b642992bb4cb0

file0.scala

package scala
abstract sealed class Boolean extends AnyVal {
  def && (p: => Boolean): Boolean = // boolean and
    if (this) p else false
  def || (p: => Boolean): Boolean = // boolean or
    if (this) true else p
  def & (x: Boolean): Boolean = // boolean strict and
    if (this) x else false
  def | (x: Boolean): Boolean = // boolean strict or
    if (this) true else x
  def == (x: Boolean): Boolean = // boolean equality
    if (this) x else x.unary_!
  def != (x: Boolean): Boolean // boolean inequality
    if (this) x.unary_! else x
  def unary_!: Boolean // boolean negation
    if (this) false else true
}

file1.scala

scala> val x = true
x: Boolean = true

scala> x.&& _
res0: Boolean => Boolean = $$Lambda$1047/1553616699@2bb0e277

scala> x.|| _
res1: Boolean => Boolean = $$Lambda$1048/2129021779@1131aead

scala> x.| _
res2: Boolean => Boolean = $$Lambda$1049/1592671657@ace2408

scala> def foo(x: => Int): Int = x
foo: (x: => Int)Int

scala> foo _
res3: (=> Int) => Int = $$Lambda$1057/1348844972@2e9dcdd3

file2.txt

Note
This method uses 'short-circuit' evaluation and behaves as if it was declared as def &&(x: => Boolean): Boolean. If a evaluates to false, false is returned without evaluating b.