動作確認コード

import org.scalatest.matchers.should.Matchers
import org.scalatest.wordspec.AnyWordSpec

class ScalaMapAddPriorityTest extends AnyWordSpec with Matchers {
  "map の + method" when {
    "keyが重複" should {
      "追加したvalueで上書きされる" in {
        (Map("a" -> 1) + ("a" -> 99)).apply("a") should be(99)
      }
    }
  }

  "map の ++ method" when {
    "keyが重複" should {
      "右側のvalueで上書きされる" in {
        (Map("a" -> 1) ++ Map("a" -> 99)).apply("a") should be(99)

        (Map("a" -> 99) ++ Map("a" -> 1)).apply("a") should be(1)

        (Map("a" -> 1) ++ Map("a" -> 99) ++ Map("a" -> 555))
          .apply("a") should be(555)
      }
    }
  }
}

scala-sandbox/ScalaMapAddPriorityTest.scala at da8969f860192bf26c242aa38c4849eb9271ec21 · tksugimoto/scala-sandbox

Scaladoc

優先順位は書かれていなかった

`+`

  /** Add a key/value pair to this map.
   *  @param    key the key
   *  @param    value the value
   *  @return   A new map with the new binding added to this map
   */
  override def updated [V1 >: V](key: K, value: V1): Map[K, V1]
  def + [V1 >: V](kv: (K, V1)): Map[K, V1]

`++`

  /** Adds a number of elements provided by a traversable object
   *  and returns a new collection with the added elements.
   *
   *  @param xs      the traversable object consisting of key-value pairs.
   *  @return        a new immutable map with the bindings of this map and those from `xs`.
   */
  override def ++[V1 >: V](xs: GenTraversableOnce[(K, V1)]): immutable.Map[K, V1] =
    ((repr: immutable.Map[K, V1]) /: xs.seq) (_ + _)

+ が呼ばれているので、+ 依存

※ /: は foldLeft

  def /:[B](z: B)(op: (B, A) => B): B = foldLeft(z)(op)

未確認事項

具体的な + 実装
- Map の実装によるのかもしれない