1 files changed, 63 insertions, 0 deletions
diff --git a/tests/disabled/long-running/t6253c.scala b/tests/disabled/long-running/t6253c.scala
new file mode 100644
index 000000000..39c3a5b17
--- /dev/null
+++ b/tests/disabled/long-running/t6253c.scala
@@ -0,0 +1,63 @@
+import scala.collection.immutable.HashSet
+
+object Test extends dotty.runtime.LegacyApp {
+
+  var hashCount = 0
+
+  /**
+   * A key that produces lots of hash collisions, to exercise the part of the code that deals with those
+   */
+  case class Collision(value: Int) {
+
+    override def hashCode = {
+      hashCount += 1
+      value / 5
+    }
+  }
+
+  /**
+   * A key that is identical to int other than that it counts hashCode invocations
+   */
+  case class HashCounter(value: Int) {
+
+    override def hashCode = {
+      hashCount += 1
+      value
+    }
+  }
+
+  def testDiff[T](sizes: Seq[Int], offsets: Seq[Double], keyType: String, mkKey: Int => T): Unit = {
+    for {
+      i <- sizes
+      o <- offsets
+    } {
+      val e = HashSet.empty[T]
+      val j = (i * o).toInt
+      // create two sets of size i with overlap o
+      val a = e ++ (0 until i).map(mkKey)
+      require(a.size == i, s"Building HashSet of size $i failed. Key type $keyType.")
+      val b = e ++ (j until (i + j)).map(mkKey)
+      require(b.size == i, s"Building HashSet of size $i failed. Key type $keyType.")
+      val as = e ++ (0 until j).map(mkKey)
+      require(as.size == j, s"Building HashSet of size $j failed. Key type $keyType.")
+      val hashCount0 = hashCount
+      val u = a diff b
+      require(hashCount == hashCount0, s"key.hashCode should not be called, but has been called ${hashCount - hashCount0} times. Key type $keyType.")
+      require(u == (a diff scala.collection.mutable.HashSet(b.toSeq: _*)), s"Operation must still work for other sets!")
+      require(u.size == j, s"Expected size $j. Real size ${u.size}. Key type $keyType.")
+      for (x <- 0 until j)
+        require(u.contains(mkKey(x)), s"Key type $keyType. Set (0 until ${i + j}) should contain $x but does not.")
+      require((as intersect b).isEmpty)
+      val b_as = b diff as
+      val as_b = as diff b
+      require((b_as eq b) || (b_as eq as), s"No structural sharing in b diff as. Key type $keyType, b=($j until ${i + j}) as=(0 until $j)")
+      require((as_b eq b) || (as_b eq as), s"No structural sharing in as diff b. Key type $keyType, b=($j until ${i + j}) as=(0 until $j)")
+    }
+  }
+
+  val sizes = Seq(1, 10, 100, 1000, 10000, 100000)
+  val offsets = Seq(0.0, 0.25, 0.5, 0.75, 1.0)
+  testDiff(sizes, offsets, "int", identity[Int])
+  testDiff(sizes, offsets, "hashCounter", HashCounter.apply)
+  testDiff(sizes, offsets, "collision", Collision.apply)
+}