Fix #8847: Add regression test

Fixed by #8652

Author: griggt

sbt-dotty/sbt-test/scala2-compat/i8847/build.sbt

@@ -0,0 +1,15 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = "2.13.3"
+
+lazy val `i8847-lib` = (project in file ("lib"))
+  .settings(scalaVersion := scala2Version)
+
+lazy val `i8847-test` = (project in file ("main"))
+  .dependsOn(`i8847-lib`)
+  .settings(
+    scalaVersion := scala3Version,
+    // https://github.com/sbt/sbt/issues/5369
+    projectDependencies := {
+      projectDependencies.value.map(_.withDottyCompat(scalaVersion.value))
+    }
+  )

sbt-dotty/sbt-test/scala2-compat/i8847/lib/i8847-lib.scala

@@ -0,0 +1,19 @@
+package i8847
+
+trait P {
+  // The `private` modifier is needed, and `X` must be a case class or case object
+  private case class X(x: Int)
+  private case object Y
+}
+
+trait Foo {
+  def foo(): Unit = ???
+}
+
+trait Bar extends Foo {
+  override def foo(): Unit = super.foo()
+}
+
+trait Baz extends Foo {
+  override def foo(): Unit = super.foo()
+}

sbt-dotty/sbt-test/scala2-compat/i8847/main/i8847-test.scala

@@ -0,0 +1,5 @@
+import i8847.{P, Bar, Baz}
+
+class T extends P
+
+class C extends Bar with Baz

sbt-dotty/sbt-test/scala2-compat/i8847/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/i8847/test

@@ -0,0 +1 @@
+> i8847-test/compile