Add check for right Tests traits in ScalaJS and Native

scalajslib/test/src/mill/scalajslib/HelloJSWorldTests.scala

@@ -62,6 +62,13 @@ object HelloJSWorldTests extends TestSuite {
       }
     }
 
+    object scalaTestsError extends ScalaJSModule {
+      val (scala, scalaJS) = matrix.head
+      def scalaVersion = scala
+      def scalaJSVersion = scalaJS
+      object test extends ScalaTests with TestModule.Utest
+    }
+
     object buildScalaTest extends Cross[BuildModuleScalaTest](matrix)
     trait BuildModuleScalaTest extends RootModule {
       object test extends ScalaJSTests with TestModule.ScalaTest {
@@ -272,6 +279,15 @@ object HelloJSWorldTests extends TestSuite {
       )
     }
 
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        HelloJSWorld.scalaTestsError.test
+      }
+      assert(
+        error.getCause.getMessage == s"scalaTestsError is a `ScalaJSModule`. scalaTestsError.test needs to extend `ScalaJSTests` instead of `ScalaTests`"
+      )
+    }
+
     def checkRun(scalaVersion: String, scalaJSVersion: String): Unit = {
       val task = HelloJSWorld.helloJsWorld(scalaVersion, scalaJSVersion).run()
 

scalalib/src/mill/scalalib/ScalaModule.scala

@@ -20,6 +20,29 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase { outer =>
   type ScalaModuleTests = ScalaTests
 
   trait ScalaTests extends JavaModuleTests with ScalaModule {
+    try {
+      if (
+        Class.forName("mill.scalajslib.ScalaJSModule").isInstance(outer) && !Class.forName(
+          "mill.scalajslib.ScalaJSModule$ScalaJSTests"
+        ).isInstance(this)
+      ) throw new Exception(
+        s"$outer is a `ScalaJSModule`. $this needs to extend `ScalaJSTests` instead of `ScalaTests`"
+      )
+    } catch {
+      case _: ClassNotFoundException => // if we can't find the classes, we certainly are not in a ScalaJSModule
+    }
+    try {
+      if (
+        Class.forName("mill.scalanativelib.ScalaNativeModule").isInstance(outer) && !Class.forName(
+          "mill.scalanativelib.ScalaNativeModule$ScalaNativeTests"
+        ).isInstance(this)
+      ) throw new Exception(
+        s"$outer is a `ScalaNativeModule`. $this needs to extend `ScalaNativeTests` instead of `ScalaTests`"
+      )
+    } catch {
+      case _: ClassNotFoundException => // if we can't find the classes, we certainly are not in a ScalaNativeModule
+    }
+
     override def scalaOrganization: Target[String] = outer.scalaOrganization()
     override def scalaVersion: Target[String] = outer.scalaVersion()
     override def scalacPluginIvyDeps: Target[Agg[Dep]] = outer.scalacPluginIvyDeps()

scalanativelib/test/src/mill/scalanativelib/HelloNativeWorldTests.scala

@@ -68,6 +68,14 @@ object HelloNativeWorldTests extends TestSuite {
         )
       }
     }
+
+    object scalaTestsError extends ScalaNativeModule {
+      val (scala, scalaNative, _) = matrix.head
+      def scalaVersion = scala
+      def scalaNativeVersion = scalaNative
+      object test extends ScalaTests with TestModule.Utest
+    }
+
     object inherited extends ScalaNativeModule {
       val (scala, scalaNative, _) = matrix.head
       def scalacOptions = Seq("-deprecation")
@@ -230,6 +238,15 @@ object HelloNativeWorldTests extends TestSuite {
       testAllMatrix((scala, scalaNative, releaseMode) => checkRun(scala, scalaNative, releaseMode))
     }
 
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        HelloNativeWorld.scalaTestsError.test
+      }
+      assert(
+        error.getCause.getMessage == s"scalaTestsError is a `ScalaNativeModule`. scalaTestsError.test needs to extend `ScalaNativeTests` instead of `ScalaTests`"
+      )
+    }
+
     def checkInheritedTargets[A](target: ScalaNativeModule => T[A], expected: A) = {
       val Right((mainResult, _)) = helloWorldEvaluator(target(HelloNativeWorld.inherited))
       val Right((testResult, _)) = helloWorldEvaluator(target(HelloNativeWorld.inherited.test))