Make sure tasty-lib doesn't warn about Scala 3 Next (#2775)

build.sc

@@ -1163,7 +1163,7 @@ trait TastyLib extends ScalaCliCrossSbtModule
          |
          |/** Build-time constants. Generated by mill. */
          |object Constants {
-         |  def latestSupportedScala = "${Scala.defaultInternal}"
+         |  def latestSupportedScala = "${Scala.defaultUser}"
          |}
          |""".stripMargin
     if (!os.isFile(dest) || os.read(dest) != code)

modules/integration/src/test/scala/scala/cli/integration/CompileTestDefinitions.scala

@@ -697,4 +697,15 @@ abstract class CompileTestDefinitions
       expect(out.contains("Too small maximum heap"))
     }
   }
+
+  test(s"TASTY processor does not warn about Scala $actualScalaVersion") {
+    TestInputs(os.rel / "simple.sc" -> s"""println("Hello")""")
+      .fromRoot { root =>
+        val result =
+          os.proc(TestUtil.cli, "compile", ".", extraOptions)
+            .call(cwd = root, stderr = os.Pipe)
+        expect(result.exitCode == 0)
+        expect(!result.err.text().contains("cannot post process TASTY files"))
+      }
+  }
 }

modules/tasty-lib/src/main/scala/scala/build/tastylib/TastyVersions.scala

@@ -1,30 +1,41 @@
 package scala.build.tastylib
 
 import scala.build.tastylib.internal.Constants
+import scala.util.Try
 
 object TastyVersions {
+  implicit class VersionOps(version: String) {
+    def majorVersion: Int               = version.split('.')(0).toInt
+    def minorVersion: Int               = version.split('.')(1).toInt
+    def minorVersionOption: Option[Int] = Try(minorVersion).toOption
+  }
 
   // Every time tasty version is updated, please update LatestSupportedScala as well!
-  object LatestSupportedScala {
-    final val MajorVersion: Int = 3
-    final val MinorVersion: Int = Constants.latestSupportedScala.split('.')(1).toInt
+  private object LatestSupportedScala {
+    final val MajorVersion: Int = Constants.latestSupportedScala.majorVersion
+    final val MinorVersion: Int = Constants.latestSupportedScala.minorVersion
+
+    def isLatestSupportedMajorVersion(scalaVersion: String): Boolean =
+      scalaVersion.startsWith(s"${LatestSupportedScala.MajorVersion}.") ||
+      scalaVersion == LatestSupportedScala.MajorVersion.toString
   }
 
   def shouldRunPreprocessor(
     scalaVersion: String,
     scalaCliVersion: String
   ): Either[String, Boolean] =
-    if (!scalaVersion.startsWith("3.") && scalaVersion != "3") Right(false)
-    else
-      scalaVersion.split('.')(1).toInt match {
-        case scalaMinor if scalaMinor > LatestSupportedScala.MinorVersion =>
-          Left(
-            s"Scala CLI (v. $scalaCliVersion) cannot post process TASTY files from Scala $scalaVersion.\n" +
-              s"This is not a fatal error since post processing only cleans up source paths in TASTY file " +
-              s"and it should not affect your application.\n" +
-              s"To get rid of this message, please update Scala CLI version."
-          )
-        case _ =>
-          Right(true)
-      }
+    if (!LatestSupportedScala.isLatestSupportedMajorVersion(scalaVersion)) Right(false)
+    else scalaVersion.minorVersionOption match {
+      case Some(scalaMinor) if scalaMinor > LatestSupportedScala.MinorVersion =>
+        Left(
+          s"""Scala CLI (v$scalaCliVersion) cannot post process TASTY files from Scala $scalaVersion.
+             |This is not a fatal error since post processing only cleans up source paths in TASTY file.
+             |It should not affect your application.
+             |You may be getting this warning because you are using a newer version of Scala than the one supported by Scala CLI (v$scalaCliVersion).
+             |Make sure your Scala CLI is up-to-date.
+             |You may need to wait for $scalaVersion support in a future version of Scala CLI.
+             |""".stripMargin
+        )
+      case _ => Right(true)
+    }
 }