better error reporting when semanticdb-scalac is not found

project/Dependencies.scala

@@ -2,7 +2,7 @@ import sbt._
 
 object Dependencies {
   val x = List(1) // scalafix:ok
-  def scalafixVersion: String = "0.11.0+118-1b88e1f8-SNAPSHOT"
+  def scalafixVersion: String = "0.11.0+132-89b843d2-SNAPSHOT"
 
   val all = List(
     "org.eclipse.jgit" % "org.eclipse.jgit" % "5.13.2.202306221912-r",

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -15,6 +15,7 @@ import scalafix.internal.sbt._
 
 import scala.collection.JavaConverters.collectionAsScalaIterableConverter
 import scala.util.control.NoStackTrace
+import sbt.librarymanagement.ResolveException
 
 object ScalafixPlugin extends AutoPlugin {
   override def trigger: PluginTrigger = allRequirements
@@ -164,6 +165,37 @@ object ScalafixPlugin extends AutoPlugin {
         SettingKey[Boolean]("bspEnabled") := false
       )
     ),
+    update := {
+      object SemanticdbScalac {
+        def unapply(id: ModuleID): Option[String] =
+          if (
+            id.organization == scalafixSemanticdb.organization &&
+            id.name.startsWith(scalafixSemanticdb.name)
+          ) Some(id.revision)
+          else None
+      }
+
+      update.result.value match {
+        case Value(v) => v
+        case Inc(inc: Incomplete) =>
+          Incomplete.allExceptions(inc).toList match {
+            case (resolveException: ResolveException) :: Nil =>
+              resolveException.failed.headOption match {
+                case Some(SemanticdbScalac(rev)) =>
+                  val scalaV = scalaVersion.value
+                  val msg = s"The SemanticDB scalac plugin version ${rev} set up " +
+                    "via `semanticdbVersion` does follow the version recommended " +
+                    "for Scalafix, but is not supported for the outdated Scala " +
+                    s"version ${scalaV}. Please upgrade to a more recent Scala " +
+                    "patch version or uninstall sbt-scalafix."
+                  throw inc.copy(message = Some(msg))
+                case _ =>
+              }
+            case _ =>
+          }
+          throw inc
+      }
+    },
     ivyConfigurations += ScalafixConfig,
     scalafixAll := scalafixAllInputTask.evaluated
   )

src/sbt-test/sbt-1.5/testkit/build.sbt

@@ -5,6 +5,7 @@ lazy val scala3Version = "3.0.0"
 
 inThisBuild(
   List(
+    scalaVersion := V.scala212, // don't let tests project fall back to sbt's old default
     semanticdbEnabled := true,
     semanticdbVersion := scalafixSemanticdb.revision,
     // need for sbt <1.7.0 as includePluginResolvers has no effect, see https://github.com/sbt/sbt/pull/6853

src/sbt-test/sbt-scalafix/basic/build.sbt

@@ -1,6 +1,6 @@
 inThisBuild(
   List(
-    scalaVersion := "2.12.14",
+    scalaVersion := "2.12.18",
     libraryDependencies ++= List(
       "org.scalameta" %% "testkit" % "4.3.10" % Test,
       "org.scalameta" %% "munit" % "0.7.9" % Test,

src/sbt-test/sbt-scalafix/inconfig/build.sbt

@@ -1,6 +1,6 @@
 inThisBuild(
   List(
-    scalaVersion := "2.12.14",
+    scalaVersion := "2.12.18",
     libraryDependencies ++= List(
       "org.scalameta" %% "testkit" % "4.3.10" % Test,
       "org.scalameta" %% "munit" % "0.7.9" % Test,

src/sbt-test/sbt-scalafix/root-validation/build.sbt

@@ -1,6 +1,6 @@
 inThisBuild(
   List(
-    scalaVersion := "2.12.14"
+    scalaVersion := "2.12.18"
   )
 )
 

src/sbt-test/sbt-scalafix/semanticdb-enabled/build.sbt

@@ -1,6 +1,6 @@
 inThisBuild(
   List(
-    scalaVersion := "2.12.14",
+    scalaVersion := "2.12.18",
     semanticdbEnabled := true,
     semanticdbVersion := scalafixSemanticdb.revision
   )

src/sbt-test/sbt-scalafix/unavailable-semanticdb-scalac/build.sbt

@@ -0,0 +1,22 @@
+inThisBuild(
+  List(
+    semanticdbEnabled := true,
+    semanticdbVersion := scalafixSemanticdb.revision,
+    scalaVersion := "2.13.7" // last semanticdb-scalac published for that scala version is 4.8.4
+  )
+)
+
+lazy val checkLogs =
+  taskKey[Unit]("Check presence of call to action in update logs")
+
+checkLogs := {
+  val taskStreams = (update / streams).value
+  val reader = taskStreams.readText(taskStreams.key)
+  val logLines = Stream
+    .continually(reader.readLine())
+    .takeWhile(_ != null)
+    .force
+  assert(
+    logLines.exists(_.contains("Please upgrade to a more recent Scala patch version or uninstall sbt-scalafix"))
+  )
+}

src/sbt-test/sbt-scalafix/unavailable-semanticdb-scalac/project/plugins.sbt

@@ -0,0 +1,2 @@
+resolvers += Resolver.sonatypeRepo("public")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % sys.props("plugin.version"))

src/sbt-test/sbt-scalafix/unavailable-semanticdb-scalac/test

@@ -0,0 +1,2 @@
+-> scalafix
+> checkLogs

src/sbt-test/skip-java17/scalafixResolvers/build.sbt

@@ -1,4 +1,4 @@
-scalaVersion := "2.12.7"
+scalaVersion := "2.12.18"
 
 TaskKey[Unit]("check") := {
   val expectedRepositories: Seq[String] = Seq(