[2.x] fix: Eviction error for downgrade of Scala 3.x (#6694)

**Problem**
When scalaVersion is Scala 3.x and a dependency brings a newer
scala3-library_3, sbt did not report an eviction error or warning.
The compiler could be older than the standard library on the
classpath, breaking compile-time alignment (e.g. scala/scala3#25406).

**Solution**
- In Compiler.scala, add an else if ScalaArtifacts.isScala3(sv) branch
  in scalaInstanceConfigFromUpdate that finds scala3-library_* on the
  Compile report and, if scalaVersion < that revision, fails (or warns
  when allowUnsafeScalaLibUpgrade := true) with the same pattern as
  the existing Scala 2.13 check (PR 7480). Message uses compile-time
  alignment wording and "See evicted to know why ... was upgraded from".
- Set allowUnsafeScalaLibUpgrade := true on b3 in stdlib-unfreeze so
  existing b3/run and b3/checkScala still pass.
- Add scripted tests: stdlib-unfreeze-scala3-eviction (expect compile
  to fail) and stdlib-unfreeze-scala3-warn (expect success with warning).

Closes #6694

main/src/main/scala/sbt/internal/Compiler.scala

@@ -215,6 +215,39 @@ object Compiler:
           if err then sys.error(msg)
           else s.log.warn(msg)
         else ()
+    else if ScalaArtifacts.isScala3(sv) then
+      val scala3LibOpt = for
+        compileReport <- fullReport.configuration(Configurations.Compile)
+        lib <- compileReport.modules.find(
+          _.module.name.startsWith(ScalaArtifacts.Scala3LibraryPrefix)
+        )
+      yield lib
+      for lib <- scala3LibOpt do
+        val libVer = lib.module.revision
+        val libName = lib.module.name
+        val proj =
+          Def.displayBuildRelative(Keys.thisProjectRef.value.build, Keys.thisProjectRef.value)
+        if VersionNumber(sv).matchesSemVer(SemanticSelector(s"<$libVer")) then
+          val err = !Keys.allowUnsafeScalaLibUpgrade.value
+          val fix =
+            if err then
+              """Upgrade the `scalaVersion` to fix the build. If upgrading the Scala compiler version is
+                |not possible (for example due to a regression in the compiler or a missing dependency),
+                |this error can be demoted by setting `allowUnsafeScalaLibUpgrade := true`.""".stripMargin
+            else s"""Note that the dependency classpath and the runtime classpath of your project
+                 |contain the newer $libName $libVer, even if the scalaVersion is $sv.
+                 |Compilation (macro expansion) or using the Scala REPL in sbt may fail with a LinkageError.""".stripMargin
+          val msg =
+            s"""Expected `$proj scalaVersion` to be $libVer or later, but found $sv.
+               |To maintain the compile-time alignment, the Scala compiler cannot be older than $libName on the dependency classpath.
+               |
+               |$fix
+               |
+               |See `$proj evicted` to know why $libName was upgraded from $sv to $libVer.
+               |""".stripMargin
+          if err then sys.error(msg)
+          else s.log.warn(msg)
+        else ()
     else ()
     def file(id: String): Option[File] =
       for

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-eviction/build.sbt

@@ -0,0 +1,12 @@
+// Regression test for #6694: Scala 3 eviction error when scalaVersion < scala3-library_3 on classpath.
+// low has scalaVersion 3.3.2 and depends on high (3.3.4), so resolved scala3-library_3 is 3.3.4.
+// Without allowUnsafeScalaLibUpgrade, compile must fail with the eviction error.
+
+lazy val high = project.settings(
+  scalaVersion := "3.3.4",
+)
+
+lazy val low = project.dependsOn(high).settings(
+  scalaVersion := "3.3.2",
+  // do NOT set allowUnsafeScalaLibUpgrade — we expect the build to fail
+)

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-eviction/high/H.scala

@@ -0,0 +1 @@
+object H { def id[A](a: A): A = a }

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-eviction/low/L.scala

@@ -0,0 +1 @@
+object L { def main(args: Array[String]): Unit = println(H.id(1)) }

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-eviction/test

@@ -0,0 +1,2 @@
+# Expect compile to fail: scalaVersion 3.3.2 < scala3-library_3 3.3.4 from high
+-> low/compile

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-warn/build.sbt

@@ -0,0 +1,11 @@
+// Same as stdlib-unfreeze-scala3-eviction but with allowUnsafeScalaLibUpgrade := true on low.
+// low has scalaVersion 3.3.2 and depends on high (3.3.4); we demote the eviction to a warning so compile succeeds.
+
+lazy val high = project.settings(
+  scalaVersion := "3.3.4",
+)
+
+lazy val low = project.dependsOn(high).settings(
+  allowUnsafeScalaLibUpgrade := true,
+  scalaVersion := "3.3.2",
+)

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-warn/high/H.scala

@@ -0,0 +1 @@
+object H { def id[A](a: A): A = a }

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-warn/low/L.scala

@@ -0,0 +1 @@
+object L { def main(args: Array[String]): Unit = println(H.id(1)) }

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze-scala3-warn/test

@@ -0,0 +1,3 @@
+# With allowUnsafeScalaLibUpgrade := true, compile and run succeed (eviction is a warning only)
+> low/compile
+> low/run

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze/build.sbt

@@ -16,6 +16,7 @@ lazy val a3 = project.settings(
 )
 
 lazy val b3 = project.dependsOn(a3).settings(
+  allowUnsafeScalaLibUpgrade := true, // b3 has 3.3.2, a3 brings 3.3.4; demote to warn so b3/run still passes
   scalaVersion := "3.3.2", // 2.13.12 library
   TaskKey[Unit]("checkScala") := {
     val i = scalaInstance.value