workaround: comment out lines to compile

algebird-core/src/main/scala-3/com/twitter/algebird/AdaptiveVector.scala

@@ -176,7 +176,7 @@ sealed trait AdaptiveVector[V] extends IndexedSeq[V] {
 
   /** How many items are not sparse */
   def denseCount: Int
-  def size: Int
+  // def size: Int
   override def apply(idx: Int): V
   def updated(idx: Int, v: V): AdaptiveVector[V]
 

algebird-core/src/main/scala-3/com/twitter/algebird/compat.scala

@@ -31,6 +31,7 @@ private[algebird] trait CompatFold {
 private[algebird] trait CompatDecayedVector {
   // This is the default monoid that never thresholds.
   // If you want to set a specific accuracy you need to implicitly override this
-  implicit def monoid[F, C[_]](implicit vs: VectorSpace[F, C], metric: Metric[C[F]]):Monoid[DecayedVector[C]] =
-    DecayedVector.monoidWithEpsilon(-1.0)
+//  implicit def monoid[F, C[_]](implicit vs: VectorSpace[F, C], metric: Metric[C[F]]):Monoid[DecayedVector[C]] =
+//    DecayedVector.monoidWithEpsilon(-1.0)
 }
+

build.sbt

@@ -39,6 +39,8 @@ def scalaVersionSpecificJavaFolders(srcBaseDir: java.io.File, scalaVersion: Stri
       new java.io.File(s"${srcBaseDir.getPath}-2.12-") :: Nil
     case Some((2, y)) if y >= 13 =>
       new java.io.File(s"${srcBaseDir.getPath}-2.13+") :: Nil
+    case Some((3, _)) =>
+      new java.io.File(s"${srcBaseDir.getPath}-2.13+") :: Nil
     case _ => Nil
   }
 
@@ -128,6 +130,10 @@ val sharedSettings = Seq(
   Compile / unmanagedSourceDirectories ++= scalaVersionSpecificJavaFolders(
     (Compile / javaSource).value,
     scalaVersion.value
+  ),
+  Test / unmanagedSourceDirectories ++= scalaVersionSpecificJavaFolders(
+    (Test / scalaSource).value,
+    scalaVersion.value
   )
 ) ++ mimaSettings
 // NOTE: After dropping Scala 2.11, we can remove src/main/scala-2.11 and share sources between scala 2.12, 2.13 and 3.x.