From ebc11f04a74ac351bca17b1de8dd40b4ce20f29a Mon Sep 17 00:00:00 2001
From: calm <148254234+calm329@users.noreply.github.com>
Date: Mon, 12 Jan 2026 13:19:24 -0800
Subject: [PATCH] [2.x] fix: Fix updateSbtClassifiers using wrong Scala version
 for cross-built plugins (#8495)

**Problem**

When cross-building sbt plugins with explicit `scalaVersion` set in `build.sbt`, the `updateSbtClassifiers` task fails because it uses the launcher's Scala version instead of the appropriate Scala version for the target sbt version.

For example, with this configuration:
```scala
lazy val root = (project in file("."))
  .enablePlugins(SbtPlugin)
  .settings(
    scalaVersion := "2.10.7",  // Explicit for IDE support
    crossSbtVersions := Seq("0.13.17", "1.0.0"),
  )
```

Running sbt "show updateSbtClassifiers" would fail with:
Error downloading org.scala-sbt:scripted-plugin_2.12:0.13.17

It should look for scripted-plugin_2.10:0.13.17 since sbt 0.13.x uses Scala 2.10.

**Solution**

Modified sbtClassifiersTasks in Defaults.scala to:
1. Check if the project is an sbt plugin (sbtPlugin.value)
2. For plugins, derive the Scala version from pluginCrossBuild / sbtBinaryVersion using PluginCross.scalaVersionFromSbtBinaryVersion
3. For non-plugins, continue using the launcher's Scala version (original behavior)
---
 main/src/main/scala/sbt/Defaults.scala         |  8 +++++++-
 .../build.sbt                                  | 18 ++++++++++++++++++
 .../project/build.sbt                          |  1 +
 .../test                                       |  1 +
 4 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/build.sbt
 create mode 100644 sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/project/build.sbt
 create mode 100644 sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/test

diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index b7b51cc0a..1ec5c34b1 100644
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -3550,7 +3550,13 @@ object Classpaths {
           classifiersModule := Def.uncached(classifiersModuleTask.value),
           // Redefine scalaVersion and scalaBinaryVersion specifically for the dependency graph used for updateSbtClassifiers task.
           // to fix https://github.com/sbt/sbt/issues/2686
-          scalaVersion := appConfiguration.value.provider.scalaProvider.version,
+          // For sbt plugins, use the Scala version corresponding to the sbt binary version being targeted.
+          // to fix https://github.com/sbt/sbt/issues/8026
+          scalaVersion := {
+            val isPlugin = sbtPlugin.value
+            if (isPlugin) (pluginCrossBuild / scalaVersion).value
+            else appConfiguration.value.provider.scalaProvider.version
+          },
           scalaBinaryVersion := binaryScalaVersion(scalaVersion.value),
           scalaEarlyVersion := CrossVersion.earlyScalaVersion(scalaVersion.value),
           scalaOrganization := ScalaArtifacts.Organization,
diff --git a/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/build.sbt b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/build.sbt
new file mode 100644
index 000000000..6c097accc
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/build.sbt
@@ -0,0 +1,18 @@
+// Test for https://github.com/sbt/sbt/issues/8026
+// When building sbt plugins with explicit scalaVersion set,
+// updateSbtClassifiers should use the correct Scala version for the sbt version.
+
+lazy val root = (project in file("."))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    name := "test-sbt-cross-build",
+
+    // Explicitly set scala version - this is what caused the issue in #8026
+    // When scalaVersion is explicitly set, updateSbtClassifiers was using the
+    // launcher's Scala version instead of the plugin's Scala version.
+    // Before the fix, this would fail with:
+    //   Error downloading org.scala-sbt:scripted-plugin_2.12:0.13.17
+    // because it used the launcher's Scala version (2.12) instead of the
+    // plugin's target Scala version derived from sbt binary version.
+    scalaVersion := "2.12.21",
+  )
diff --git a/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/project/build.sbt b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/project/build.sbt
new file mode 100644
index 000000000..b090aa9d7
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/project/build.sbt
@@ -0,0 +1 @@
+ThisBuild / pluginCrossBuild / sbtVersion := "2.0.0"
diff --git a/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/test b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/test
new file mode 100644
index 000000000..00ad751bf
--- /dev/null
+++ b/sbt-app/src/sbt-test/dependency-management/i8026-update-sbt-classifiers-cross-plugin/test
@@ -0,0 +1 @@
+> updateSbtClassifiers