In dc6680b a cast is added to prevent copying elements.
In scala 2.12 this is allowed, but in scala 2.13 (with the new collections API) this is incompatible.

import scala.collection.mutable.ArrayBuffer
import scala.collection.immutable.IndexedSeq

val f = new ArrayBuffer[Int]
f.asInstanceOf[IndexedSeq[Int]]

fails with java.lang.ClassCastException: class scala.collection.mutable.ArrayBuffer cannot be cast to class scala.collection.immutable.IndexedSeq (scala.collection.mutable.ArrayBuffer and scala.collection.immutable.IndexedSeq

The builds of this commit also indicate that the 2.13 finagle-core fails since.

It seems btw that no code paths hit this, but the unit tests fail.