From 4fd24198d050b5b87457ac45b1fee79ea34b3980 Mon Sep 17 00:00:00 2001
From: Matt Dziuban <mrdziuban@gmail.com>
Date: Wed, 20 May 2026 17:56:26 -0400
Subject: [PATCH] Use a mutable Set for `alreadySeen`.

---
 main/src/main/scala/sbt/internal/IncrementalTest.scala | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/main/src/main/scala/sbt/internal/IncrementalTest.scala b/main/src/main/scala/sbt/internal/IncrementalTest.scala
index 01d227cd9..0a9faaa7b 100644
--- a/main/src/main/scala/sbt/internal/IncrementalTest.scala
+++ b/main/src/main/scala/sbt/internal/IncrementalTest.scala
@@ -192,7 +192,7 @@ class ClassStamper private[sbt] (
   ): Option[Digest] =
     val builder = Set.newBuilder[Digest]
     analyses.foreach: analysis =>
-      internalStamp(builder, javaClassName, analysis, Set.empty, log)
+      internalStamp(builder, javaClassName, analysis, mutable.Set.empty, log)
     val digests = builder.result().toSeq.sorted
     // log.debug(s"test: transitiveStamp($javaClassName, $extraHashes) = $digests")
     if digests.nonEmpty then Some(Digest.sha256Hash(digests ++ extraHashes*))
@@ -202,7 +202,7 @@ class ClassStamper private[sbt] (
       builder: mutable.Builder[Digest, Set[Digest]],
       javaClassName: String,
       analysis: Analysis,
-      alreadySeen: Set[String],
+      alreadySeen: mutable.Set[String],
       log: Logger,
   ): Unit =
     import analysis.relations
@@ -216,7 +216,7 @@ class ClassStamper private[sbt] (
       relations
         .internalClassDeps(className)
         .foreach: otherCN =>
-          internalStamp(newBuilder, otherCN, analysis, alreadySeen + javaClassName, log)
+          internalStamp(newBuilder, otherCN, analysis, alreadySeen, log)
       // log.debug(s"  internalStamp: internalDeps: $className = $internalDeps")
       relations
         .externalDeps(className)
@@ -247,7 +247,8 @@ class ClassStamper private[sbt] (
     else
       stamps.get(javaClassName) match
         case Some(xs) => builder ++= xs
-        case _        =>
+        case _ =>
+          alreadySeen += javaClassName
           // Note: internalClassDeps uses Scala-encoded class name for companion objects
           val classNames = relations.productClassName.reverse(javaClassName)
           classNames.foreach(internalStamp0)