Let's see if this fixes classpath issues.

Author: Quafadas

scalajslib/src/mill/scalajslib/ScalaJSModule.scala

@@ -74,8 +74,13 @@ trait ScalaJSModule extends scalalib.ScalaModule { outer =>
   def scalaJSLinkerClasspath: T[Loose.Agg[PathRef]] = T {
     val commonDeps = Seq(
       ivy"org.scala-js::scalajs-sbt-test-adapter:${scalaJSVersion()}",
-      ivy"com.armanbilge::scalajs-importmap:0.1.1"
+      
     )
+    val maybeImportMap = scalaJSVersion() match {
+      case s"1.$n.$_" if n.toIntOption.exists(_ < 16) => Seq[Dep]()
+      case _ => Seq(ivy"com.armanbilge::scalajs-importmap:0.1.1")
+    }
+    
     val envDeps = scalaJSBinaryVersion() match {
       case "0.6" =>
         Seq(
@@ -90,13 +95,13 @@ trait ScalaJSModule extends scalalib.ScalaModule { outer =>
     // we need to use the scala-library of the currently running mill
     resolveDependencies(
       repositoriesTask(),
-      (commonDeps.iterator ++ envDeps)
+      (commonDeps.iterator ++ envDeps ++ maybeImportMap)
         .map(Lib.depToBoundDep(_, mill.main.BuildInfo.scalaVersion, "")),
       ctx = Some(T.log)
     )
   }
 
-  def scalaJSToolsClasspath = T { scalaJSWorkerClasspath() ++ scalaJSLinkerClasspath() }
+  def scalaJSToolsClasspath = T { scalaJSWorkerClasspath() ++ scalaJSLinkerClasspath()}
 
   def fastLinkJS: Target[Report] = T.persistent {
     linkTask(isFullLinkJS = false, forceOutJs = false)()
@@ -268,9 +273,11 @@ trait ScalaJSModule extends scalalib.ScalaModule { outer =>
 
   def moduleSplitStyle: Target[ModuleSplitStyle] = T { ModuleSplitStyle.FewestModules }
 
-  def scalaJSOptimizer: Target[Boolean] = T { true }
+  def scalaJSOptimizer: Target[Boolean] = T { true }  
 
-  def esModuleRemap: Target[Map[String, String]] = T { Map.empty[String, String] }
+  def esModuleRemap: Target[Map[String, String]] = T {    
+    Map.empty[String, String] 
+  }
 
   /** Whether to emit a source map. */
   def scalaJSSourceMap: Target[Boolean] = T { true }

scalajslib/test/src/mill/scalajslib/RemapEsModuleTests.scala

@@ -25,6 +25,18 @@ object EsModuleRemapTests extends TestSuite {
       )
     }
 
+    object OldJsModule extends ScalaJSModule {
+      override def millSourcePath = workspacePath
+      override def scalaVersion = sys.props.getOrElse("TEST_SCALA_2_13_VERSION", ???)
+      override def scalaJSVersion = "1.15.0"
+      override def scalaJSSourceMap = false
+      override def moduleKind = ModuleKind.ESModule
+
+      override def esModuleRemap: Target[Map[String, String]] = Map(
+        "@stdlib/linspace" -> remapTo
+      )
+    }
+
     override lazy val millDiscover = Discover[this.type]
   }
 
@@ -47,6 +59,13 @@ object EsModuleRemapTests extends TestSuite {
       val rawJs = os.read.lines(mainPath)
       assert(rawJs(1).contains(remapTo))
     }
+
+    test("should throw for older scalaJS versions") {
+      val Left(ex) = evaluator(EsModuleRemap.OldJsModule.fastLinkJS)
+      val error = ex.asFailing.get.toString()      
+      assert(error.contains("will work with scalaJS 1.16 and above. You are using scalaJS 1.15.0"))
+    }
+
   }
 
   def prepareWorkspace(): Unit = {

scalajslib/worker/1/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala

@@ -209,6 +209,9 @@ class ScalaJSWorkerImpl extends ScalaJSWorkerApi {
       irFiles = if (esModuleMap.isEmpty) {
         irFiles0
       } else {
+        if (!minorIsGreaterThanOrEqual(16)) {
+          throw new Exception(s"Remapping EsModule imports will work with scalaJS 1.16 and above. You are using scalaJS ${ScalaJSVersions.current} - consider upgrading?")
+        }
         val remapFct = esModuleMap.toSeq.foldLeft((in: String) => in) { case (fct, (s1, s2)) =>
           val fct2: (String => String) = (in => in.replace(s1, s2))
           (in => fct(fct2(in)))