From 21a9996b099d994d1d1217208ecdee73e5d2ca2b Mon Sep 17 00:00:00 2001
From: Eugene Yokota <eed3si9n@gmail.com>
Date: Mon, 25 Aug 2025 05:16:10 -0400
Subject: [PATCH] fix: Java-only tests

**Problem**
managedScalaInstance := false fails Java-only tests because
the test runner is based on a Scala Runner and sbt 2.x
currently derives the ScalaInstance only from update.

**Solution**
For the purpose of Java-only testing, create a dummy Scala Instance.
---
 .../main/scala/sbt/internal/Compiler.scala    | 22 +++++++++++++++++--
 .../cache-resolver/{pending => test}          |  0
 .../deliver-artifacts/{pending => test}       |  0
 .../test-artifact/{pending => test}           |  0
 .../java/no-scala-tool/{pending => test}      |  0
 .../sbt-test/tests/junit-java-only/build.sbt  |  2 ++
 .../src/test/java/com/example/ATest.java      |  9 ++++++++
 .../src/sbt-test/tests/junit-java-only/test   |  2 ++
 sbt-app/src/sbt-test/tests/junit/build.sbt    |  2 +-
 9 files changed, 34 insertions(+), 3 deletions(-)
 rename sbt-app/src/sbt-test/dependency-management/cache-resolver/{pending => test} (100%)
 rename sbt-app/src/sbt-test/dependency-management/deliver-artifacts/{pending => test} (100%)
 rename sbt-app/src/sbt-test/dependency-management/test-artifact/{pending => test} (100%)
 rename sbt-app/src/sbt-test/java/no-scala-tool/{pending => test} (100%)
 create mode 100644 sbt-app/src/sbt-test/tests/junit-java-only/build.sbt
 create mode 100644 sbt-app/src/sbt-test/tests/junit-java-only/src/test/java/com/example/ATest.java
 create mode 100644 sbt-app/src/sbt-test/tests/junit-java-only/test

diff --git a/main/src/main/scala/sbt/internal/Compiler.scala b/main/src/main/scala/sbt/internal/Compiler.scala
index 6731f4a13..52eb9a25f 100644
--- a/main/src/main/scala/sbt/internal/Compiler.scala
+++ b/main/src/main/scala/sbt/internal/Compiler.scala
@@ -19,14 +19,32 @@ object Compiler:
       val sh = Keys.scalaHome.value
       val app = Keys.appConfiguration.value
       val sv = Keys.scalaVersion.value
+      val managed = Keys.managedScalaInstance.value
       sh match
         case Some(h) => scalaInstanceFromHome(h)
         case _ =>
           val scalaProvider = app.provider.scalaProvider
-          scalaInstanceFromUpdate
+          if !managed then emptyScalaInstance
+          else scalaInstanceFromUpdate
     }
 
-  // use the same class loader as the Scala classes used by sbt
+  /**
+   * A dummy ScalaInstance for Java-only projects.
+   */
+  def emptyScalaInstance: Def.Initialize[Task[ScalaInstance]] = Def.task {
+    makeScalaInstance(
+      "0.0.0",
+      Array.empty,
+      Seq.empty,
+      Seq.empty,
+      Keys.state.value,
+      Keys.scalaInstanceTopLoader.value,
+    )
+  }
+
+  // Use the same class loader as the Scala classes used by sbt
+  // This will fail for "doc" task https://github.com/sbt/sbt/issues/7725
+  // because Scala 3 uses ScalaDocTool configuration to manage doc tools.
   def optimizedScalaInstance(
       sv: String,
       scalaProvider: ScalaProvider
diff --git a/sbt-app/src/sbt-test/dependency-management/cache-resolver/pending b/sbt-app/src/sbt-test/dependency-management/cache-resolver/test
similarity index 100%
rename from sbt-app/src/sbt-test/dependency-management/cache-resolver/pending
rename to sbt-app/src/sbt-test/dependency-management/cache-resolver/test
diff --git a/sbt-app/src/sbt-test/dependency-management/deliver-artifacts/pending b/sbt-app/src/sbt-test/dependency-management/deliver-artifacts/test
similarity index 100%
rename from sbt-app/src/sbt-test/dependency-management/deliver-artifacts/pending
rename to sbt-app/src/sbt-test/dependency-management/deliver-artifacts/test
diff --git a/sbt-app/src/sbt-test/dependency-management/test-artifact/pending b/sbt-app/src/sbt-test/dependency-management/test-artifact/test
similarity index 100%
rename from sbt-app/src/sbt-test/dependency-management/test-artifact/pending
rename to sbt-app/src/sbt-test/dependency-management/test-artifact/test
diff --git a/sbt-app/src/sbt-test/java/no-scala-tool/pending b/sbt-app/src/sbt-test/java/no-scala-tool/test
similarity index 100%
rename from sbt-app/src/sbt-test/java/no-scala-tool/pending
rename to sbt-app/src/sbt-test/java/no-scala-tool/test
diff --git a/sbt-app/src/sbt-test/tests/junit-java-only/build.sbt b/sbt-app/src/sbt-test/tests/junit-java-only/build.sbt
new file mode 100644
index 000000000..f83e939f8
--- /dev/null
+++ b/sbt-app/src/sbt-test/tests/junit-java-only/build.sbt
@@ -0,0 +1,2 @@
+libraryDependencies += "com.github.sbt" % "junit-interface" % "0.13.2" % Test
+managedScalaInstance := false
diff --git a/sbt-app/src/sbt-test/tests/junit-java-only/src/test/java/com/example/ATest.java b/sbt-app/src/sbt-test/tests/junit-java-only/src/test/java/com/example/ATest.java
new file mode 100644
index 000000000..45923a214
--- /dev/null
+++ b/sbt-app/src/sbt-test/tests/junit-java-only/src/test/java/com/example/ATest.java
@@ -0,0 +1,9 @@
+package com.example;
+
+import org.junit.Test;
+
+public final class ATest {
+  @Test
+  public void testSomething() {
+  }
+}
diff --git a/sbt-app/src/sbt-test/tests/junit-java-only/test b/sbt-app/src/sbt-test/tests/junit-java-only/test
new file mode 100644
index 000000000..6b91dcb10
--- /dev/null
+++ b/sbt-app/src/sbt-test/tests/junit-java-only/test
@@ -0,0 +1,2 @@
+> test
+> doc
diff --git a/sbt-app/src/sbt-test/tests/junit/build.sbt b/sbt-app/src/sbt-test/tests/junit/build.sbt
index 92bce8545..cadc5cdae 100644
--- a/sbt-app/src/sbt-test/tests/junit/build.sbt
+++ b/sbt-app/src/sbt-test/tests/junit/build.sbt
@@ -1 +1 @@
-libraryDependencies += "com.novocode" % "junit-interface" % "0.8" % "test"
+libraryDependencies += "com.github.sbt" % "junit-interface" % "0.13.2" % Test