Merge pull request scala-js#4628 from sjrd/fix-arraybuilder-length-knownsize

Fix scala-js#4627: Correctly implement ArrayBuilder.{length,knownSize}.

Author: gzm0

scalalib/overrides-2.13/scala/collection/mutable/ArrayBuilder.scala

@@ -103,6 +103,10 @@ object ArrayBuilder {
     protected[this] def elems: Array[T] = throw new Error("unreachable")
     private var jsElems: js.Array[Any] = js.Array()
 
+    override def length: Int = jsElems.length
+
+    override def knownSize: Int = jsElems.length
+
     def addOne(elem: T): this.type = {
       val unboxedElem =
         if (isCharArrayBuilder) elem.asInstanceOf[Char].toInt

test-suite/shared/src/test/scala/org/scalajs/testsuite/scalalib/ArrayBuilderTest.scala

@@ -324,11 +324,46 @@ class ArrayBuilderTest {
     b.addAll(arr, 3, 2)
     assertArrayEquals(Array[Int](4, 5), b.result())
   }
+
+  @Test def lengthAndKnownSize_Issue4627(): Unit = {
+    assumeFalse("Needs at least Scala 2.13",
+        scalaVersion.startsWith("2.11.") ||
+        scalaVersion.startsWith("2.12."))
+
+    val b = ArrayBuilder.make[Int]
+    assertEquals(0, b.length)
+    assertEquals(0, b.knownSize)
+
+    b += 5
+    b += 7
+    assertEquals(2, b.length)
+    assertEquals(2, b.knownSize)
+
+    b += -1
+    b += 98
+    b += 4
+    assertEquals(5, b.length)
+    assertEquals(5, b.knownSize)
+
+    b.clear()
+    assertEquals(0, b.length)
+    assertEquals(0, b.knownSize)
+
+    b += 4
+    assertEquals(1, b.length)
+    assertEquals(1, b.knownSize)
+  }
 }
 
 object ArrayBuilderTest {
   implicit class ArrayBuilderCompat[A](val __sef: ArrayBuilder[A]) extends AnyVal {
     def addAll(xs: Array[_ <: A], offset: Int, length: Int): Unit =
       throw new AssertionError("unreachable code")
+
+    def length: Int =
+      throw new AssertionError("unreachable code")
+
+    def knownSize: Int =
+      throw new AssertionError("unreachable code")
   }
 }