Add check for right Tests traits in ScalaJS and Native

scalajslib/test/src/mill/scalajslib/ScalaTestsErrorTests.scala

@@ -0,0 +1,31 @@
+package mill.scalajslib
+
+import mill._
+import mill.define.Discover
+import mill.scalalib.TestModule
+import mill.util.TestUtil
+import utest._
+
+object ScalaTestsErrorTests extends TestSuite {
+  object ScalaTestsError extends TestUtil.BaseModule {
+    object scalaTestsError extends ScalaJSModule {
+      def scalaVersion = sys.props.getOrElse("TEST_SCALA_3_3_VERSION", ???)
+      def scalaJSVersion = sys.props.getOrElse("TEST_SCALAJS_VERSION", ???)
+      object test extends ScalaTests with TestModule.Utest
+    }
+
+    override lazy val millDiscover = Discover[this.type]
+  }
+
+  def tests: Tests = Tests {
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        ScalaTestsError.scalaTestsError.test
+      }
+      val message = error.getCause.getMessage
+      assert(
+        message == s"scalaTestsError is a `ScalaJSModule`. scalaTestsError.test needs to extend `ScalaJSTests`."
+      )
+    }
+  }
+}

scalalib/src/mill/scalalib/ScalaModule.scala

@@ -1,7 +1,15 @@
 package mill
 package scalalib
 
-import mill.api.{DummyInputStream, JarManifest, PathRef, Result, SystemStreams, internal}
+import mill.api.{
+  DummyInputStream,
+  JarManifest,
+  MillException,
+  PathRef,
+  Result,
+  SystemStreams,
+  internal
+}
 import mill.main.BuildInfo
 import mill.util.{Jvm, Util}
 import mill.util.Jvm.createJar
@@ -20,6 +28,29 @@ 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$ScalaJSTests"
+        ).isInstance(this)
+      ) throw new MillException(
+        s"$outer is a `ScalaJSModule`. $this needs to extend `ScalaJSTests`."
+      )
+    } catch {
+      case _: ClassNotFoundException => // if we can't find the classes, we certainly are not in a ScalaJSModule
+    }
+    try {
+      if (
+        Class.forName("mill.scalanativelib.ScalaNativeModule").isInstance(outer) && !Class.forName(
+          "mill.scalanativelib.ScalaNativeModule$ScalaNativeTests"
+        ).isInstance(this)
+      ) throw new MillException(
+        s"$outer is a `ScalaNativeModule`. $this needs to extend `ScalaNativeTests`."
+      )
+    } 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/ScalaTestsErrorTests.scala

@@ -0,0 +1,31 @@
+package mill.scalanativelib
+
+import mill._
+import mill.define.Discover
+import mill.scalalib.TestModule
+import mill.util.TestUtil
+import utest._
+
+object ScalaTestsErrorTests extends TestSuite {
+  object ScalaTestsError extends TestUtil.BaseModule {
+    object scalaTestsError extends ScalaNativeModule {
+      def scalaVersion = sys.props.getOrElse("TEST_SCALA_3_3_VERSION", ???)
+      def scalaNativeVersion = sys.props.getOrElse("TEST_SCALANATIVE_VERSION", ???)
+      object test extends ScalaTests with TestModule.Utest
+    }
+
+    override lazy val millDiscover = Discover[this.type]
+  }
+
+  def tests: Tests = Tests {
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        ScalaTestsError.scalaTestsError.test
+      }
+      val message = error.getCause.getMessage
+      assert(
+        message == s"scalaTestsError is a `ScalaNativeModule`. scalaTestsError.test needs to extend `ScalaNativeTests`."
+      )
+    }
+  }
+}