Merge pull request scala/scala#10226 from som-snytt/tweak/this-avoids-discard

Leverage this.type

Author: lrytz

library/src/scala/MatchError.scala

@@ -23,11 +23,11 @@ final class MatchError(@transient obj: Any) extends RuntimeException {
   private[this] lazy val objString = {
     def ofClass = "of class " + obj.getClass.getName
     if (obj == null) "null"
-    else try {
-      obj.toString() + " (" + ofClass + ")"
-    } catch {
-      case _: Throwable => "an instance " + ofClass
-    }
+    else
+      try s"$obj ($ofClass)"
+      catch {
+        case _: Throwable => "an instance " + ofClass
+      }
   }
 
   @throws[java.io.ObjectStreamException]

library/src/scala/collection/mutable/ArrayBuffer.scala

@@ -296,7 +296,8 @@ object ArrayBuffer extends StrictOptimizedSeqFactory[ArrayBuffer] {
     if (k >= 0) {
       // Avoid reallocation of buffer if length is known
       val array = ensureSize(emptyArray, 0, k) // don't duplicate sizing logic, and check VM array size limit
-      IterableOnce.copyElemsToArray(coll, array.asInstanceOf[Array[Any]])
+      val actual = IterableOnce.copyElemsToArray(coll, array.asInstanceOf[Array[Any]])
+      if (actual != k) throw new IllegalStateException(s"Copied $actual of $k")
       new ArrayBuffer[B](array, k)
     }
     else new ArrayBuffer[B] ++= coll