键入错误推断

Question

给出以下代码：

implicit class RichString(s: String) {
  def !![T](createMessage: Long => T) = ()
}
case class SomeClass(i: Int, s: String, id: Long)

为什么会这样：

"some string" !! SomeClass(5, "test", _)

不编译，抛出以下错误：

Missing parameter type for expanded function ((x$1) => "some string".$bang$bang(SomeClass(5, "test", x$1)))
 "some string" !! SomeClass(5, "test", _)
                                       ^

Type mismatch;
 found   : cmd1.SomeClass
 required: Long => cmd1.SomeClass "some string" !! SomeClass(5, "test", _)

但所有这些都编译得很好：

val f = SomeClass(5, "test", _)
"some string" !! f

"some string" !! ( SomeClass(5, "test", _) )

"some string" !! { SomeClass(5, "test", _) }

PS：此"some string" !! SomeClass(5, "test", _: Long)也会导致类型不匹配。

Answer 1

正如消息所述，"some string" !! SomeClass(5, "test", _)扩展为x => "some string" !! SomeClass(5, "test", x)，不是 "some string" !! { x => SomeClass(5, "test", x) }。因此!!没有收到函数，它会收到SomeClass，但不会编译。