当方法以看似不相关的方式重载时，为什么 scala 无法编译？

class A {}
class B extends A {}

object Sample {
  def foo(a: Set[A]) {
    println("Hi Set[A]")
  }
  // def foo(a: String) {
  //   println("Hi A")
  // }
}

Sample.foo(Set(new B()))

上面的代码可以愉快地运行scala。然而，当我取消评论时foo(a: String)，代码无法编译：

test.scala:13: error: overloaded method value foo with alternatives:
  (a: String)Unit <and>
  (a: Set[this.A])Unit
 cannot be applied to (scala.collection.immutable.Set[this.B])
Sample.foo(Set(new B()))
       ^
one error found

foo(a: String)似乎它应该与尝试打电话无关foo with a Set[B]。到底是怎么回事？

EDIT:

让我困惑的不仅是为什么未注释的版本无法编译，还在于为什么它是否编译, when foo(a: String)被注释掉了。我通过添加方法改变了什么foo(a: String)?

Set不变并不能解释为什么它在以下情况下编译成功foo(a: String)被注释掉了。

在工作情况下，类型参数为Set.apply[T]被推断为A因为Set[A]是函数参数的预期类型。

重载解析类型检查参数没有预期的类型，因此编译器不能再使用Set[A]指导推断您想要什么集合。

这是一个重要的收获the spec http://www.scala-lang.org/files/archive/spec/2.12/06-expressions.html#overloading-resolution，尽管现在它有点被更多有关 SAM 的词语所掩盖。

否则，让 Si... 成为通过键入每个获得的类型列表 论证如下。 [关于函数的一些内容。] 所有其他参数 使用未定义的预期类型键入。

如果它知道一个Set[A]是预期的，你的集合是这样输入的，而不是作为Set[B].

您可以观察打字决策-Ytyper-debug，它发出的输出有时并不难以理解。

Given

class A ; class B extends A

object X { def f(as: Set[A]) = ??? ; def f(s: String) = ??? }

object Main extends App {
  X.f(Set(new B))
}

此处，值参数的类型为Set[B]，然后它尝试找到到重载的参数类型的隐式转换，但失败了。

它还寻找一个转换X对象具有一个类型f方法采用Set[B].

|    |-- X.f(Set(new B())) BYVALmode-EXPRmode (site: value <local Main> in Main)
|    |    |-- X.f BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |-- X EXPRmode-POLYmode-QUALmode (silent: value <local Main> in Main)
|    |    |    |    \-> X.type
|    |    |    \-> (s: String)Nothing <and> (as: Set[A])Nothing
|    |    |-- Set(new B()) BYVALmode-EXPRmode (silent: value <local Main> in Main)
|    |    |    |-- Set BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- scala.Predef.Set.apply BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    [adapt] [A](elems: A*)CC[A] adapted to [A](elems: A*)CC[A]
|    |    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |    [adapt] => scala.collection.immutable.Set.type adapted to [A](elems: A*)CC[A]
|    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |-- new B() BYVALmode-EXPRmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- new B BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    |-- new B EXPRmode-POLYmode-QUALmode (silent: value <local Main> in Main)
|    |    |    |    |    |    |-- B FUNmode-TYPEmode (silent: value <local Main> in Main)
|    |    |    |    |    |    |    \-> B
|    |    |    |    |    |    \-> B
|    |    |    |    |    \-> ()B
|    |    |    |    \-> B
|    |    |    solving for (A: ?A)
|    |    |    \-> scala.collection.immutable.Set[B]
|    |    [search #1] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=scala.collection.immutable.Set[B] => String (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #2] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=(=> scala.collection.immutable.Set[B]) => String (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #3] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=scala.collection.immutable.Set[B] => Set[A] (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #4] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=(=> scala.collection.immutable.Set[B]) => Set[A] (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    second try: <error> and Set(new B())
|    |    |-- Set(new B()) EXPRmode (silent: value <local Main> in Main)
|    |    |    |-- Set BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- scala.Predef.Set.apply BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    [adapt] [A](elems: A*)CC[A] adapted to [A](elems: A*)CC[A]
|    |    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |    [adapt] => scala.collection.immutable.Set.type adapted to [A](elems: A*)CC[A]
|    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    solving for (A: ?A)
|    |    |    \-> scala.collection.immutable.Set[B]
|    |    [search #5] start `X.type`, searching for adaptation to pt=X.type => ?{def f(x$1: ? >: scala.collection.immutable.Set[B]): ?} (silent: value <local Main> in Main) implicits disabled
|    |    [search #6] start `X.type`, searching for adaptation to pt=(=> X.type) => ?{def f(x$1: ? >: scala.collection.immutable.Set[B]): ?} (silent: value <local Main> in Main) implicits disabled
badset.scala:7: error: overloaded method value f with alternatives:
  (s: String)Nothing <and>
  (as: Set[A])Nothing
 cannot be applied to (scala.collection.immutable.Set[B])