Add an error about Scala 2.12 not supported for toolkits

Author: MaciejG604

modules/build/src/test/scala/scala/build/tests/BuildTests.scala

@@ -951,4 +951,28 @@ abstract class BuildTests(server: Boolean) extends TestUtil.ScalaCliBuildSuite {
         expect(maybeBuild.exists(_.success))
       }
     }
+
+  for (dirValue <- Seq("default", "typelevel:default"))
+    test(s"error when toolkit $dirValue is used with Scala 2.12") {
+      val testInputs = TestInputs(
+        os.rel / "simple.sc" ->
+          s"""//> using toolkit $dirValue
+             |
+             |val n = 2
+             |println(s"n=$$n")
+             |""".stripMargin
+      )
+
+      val scala212Options = baseOptions.copy(
+        scalaOptions = baseOptions.scalaOptions.copy(
+          scalaVersion = Some(MaybeScalaVersion(Constants.defaultScala212Version)),
+          scalaBinaryVersion = None
+        ),
+        scriptOptions = ScriptOptions(Some(true))
+      )
+
+      testInputs.withBuild(scala212Options, buildThreads, bloopConfigOpt) { (_, _, maybeBuild) =>
+        expect(maybeBuild.left.exists(_.message.startsWith("Toolkits do not support Scala 2.12")))
+      }
+    }
 }

modules/core/src/main/scala/scala/build/errors/ToolkitVersionError.scala

@@ -0,0 +1,6 @@
+package scala.build.errors
+
+import scala.build.Position
+
+final class ToolkitVersionError(msg: String, positions: Seq[Position])
+    extends BuildException(msg, positions)

modules/options/src/main/scala/scala/build/Artifacts.scala

@@ -2,14 +2,12 @@ package scala.build
 
 import coursier.cache.FileCache
 import coursier.core.{Classifier, Module, ModuleName, Organization, Repository, Version}
-import coursier.error.{CoursierError, ResolutionError}
-import coursier.parse.RepositoryParser
+import coursier.error.ResolutionError
 import coursier.util.Task
-import coursier.{Dependency => CsDependency, Fetch, Resolution, core => csCore, util => csUtil}
+import coursier.{Fetch, Resolution, Dependency => CsDependency, core => csCore, util => csUtil}
 import dependency.*
 
 import java.net.URL
-
 import scala.build.CoursierUtils.*
 import scala.build.EitherCps.{either, value}
 import scala.build.Ops.*
@@ -18,7 +16,7 @@ import scala.build.errors.{
   CompositeBuildException,
   FetchingDependenciesError,
   NoScalaVersionProvidedError,
-  RepositoryFormatError
+  ToolkitVersionError
 }
 import scala.build.internal.Constants
 import scala.build.internal.Constants.*
@@ -654,6 +652,17 @@ object Artifacts {
     dependencies: Seq[Positioned[coursier.Dependency]],
     resolutionError: coursier.error.ResolutionError.Simple
   ) = resolutionError match {
+    // FIXME This if may not be enough when new toolkits are added
+    case ex: ResolutionError.CantDownloadModule
+        if ex.module.name.value == s"${Constants.toolkitName}_2.12" || ex.module.name.value == s"${Constants.toolkitTestName}_2.12" =>
+      val errorPositions = dependencies.collect {
+        case Positioned(pos, dep)
+            if ex.module == dep.module => pos
+      }.flatten
+      new ToolkitVersionError(
+        "Toolkits do not support Scala 2.12",
+        errorPositions
+      )
     case ex: ResolutionError.CantDownloadModule =>
       val errorPositions = dependencies.collect {
         case Positioned(pos, dep)