diff --git a/sbt-test/scala3-compat/anon-prod-mirrors-3.1/app/Main.scala b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/app/Main.scala
new file mode 100644
index 000000000000..dfdc0d952cbd
--- /dev/null
+++ b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/app/Main.scala
@@ -0,0 +1,59 @@
+import scala.deriving.Mirror
+
+package lib {
+  object NewMirrors {
+    val mFoo = summon[Mirror.Of[Foo]] // we can access the constructor of Foo here.
+    val mFooObj = summon[Mirror.Of[Foo.type]]
+
+    object SubBar extends Bar(1) {
+      val mBar = summon[deriving.Mirror.ProductOf[Bar]]
+      val mBarObj = summon[deriving.Mirror.ProductOf[Bar.type]]
+    }
+  }
+}
+
+package app {
+  object Main:
+
+    def testFoo(): Unit = {
+      val oldMirrorFoo: Mirror.ProductOf[lib.Foo] = lib.OldMirrors.mFoo
+      val oldMirrorFooObj: Mirror.ProductOf[lib.Foo.type] = lib.OldMirrors.mFooObj
+
+      assert(oldMirrorFoo eq oldMirrorFooObj) // - not good as oldMirrorFoo is really the mirror for `Foo.type`
+      assert(oldMirrorFooObj eq lib.Foo) // - object Foo is its own mirror
+
+      // 3.1 bug: mirror for Foo behaves as mirror for Foo.type
+      assert(oldMirrorFooObj.fromProduct(EmptyTuple) == lib.Foo)
+
+      val newMirrorFoo: Mirror.ProductOf[lib.Foo] = lib.NewMirrors.mFoo
+      val newMirrorFooObj: Mirror.ProductOf[lib.Foo.type] = lib.NewMirrors.mFooObj
+
+      assert(oldMirrorFooObj eq newMirrorFooObj) // mirror for Foo.type has not changed.
+
+      assert(newMirrorFoo ne lib.Foo) // anonymous mirror for Foo
+      assert(newMirrorFoo.fromProduct(Tuple(23)).x == 23) // mirror for Foo behaves as expected
+    }
+
+    def testBar(): Unit = {
+      val oldMirrorBar: Mirror.ProductOf[lib.Bar] = lib.OldMirrors.SubBar.mBar
+      val oldMirrorBarObj: Mirror.ProductOf[lib.Bar.type] = lib.OldMirrors.SubBar.mBarObj
+
+      assert(oldMirrorBar eq oldMirrorBarObj) // - not good as oldMirrorBar is really the mirror for `Bar.type`
+      assert(oldMirrorBarObj eq lib.Bar) // - object Bar is its own mirror
+
+      // 3.1 bug: mirror for Bar behaves as mirror for Bar.type
+      assert(oldMirrorBarObj.fromProduct(EmptyTuple) == lib.Bar)
+
+      val newMirrorBar: Mirror.ProductOf[lib.Bar] = lib.NewMirrors.SubBar.mBar
+      val newMirrorBarObj: Mirror.ProductOf[lib.Bar.type] = lib.NewMirrors.SubBar.mBarObj
+
+      assert(oldMirrorBarObj eq newMirrorBarObj) // mirror for Bar.type has not changed.
+
+      assert(newMirrorBar ne lib.Bar) // anonymous mirror for Bar
+      assert(newMirrorBar.fromProduct(Tuple(23)).x == 23) // mirror for Bar behaves as expected
+    }
+
+    def main(args: Array[String]): Unit =
+      testFoo()
+      testBar()
+}
diff --git a/sbt-test/scala3-compat/anon-prod-mirrors-3.1/build.sbt b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/build.sbt
new file mode 100644
index 000000000000..2e220893eae4
--- /dev/null
+++ b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/build.sbt
@@ -0,0 +1,7 @@
+lazy val lib = project.in(file("lib"))
+  .settings(
+    scalaVersion := "3.1.1"
+  )
+
+lazy val app = project.in(file("app"))
+  .dependsOn(lib)
diff --git a/sbt-test/scala3-compat/anon-prod-mirrors-3.1/lib/Foo.scala b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/lib/Foo.scala
new file mode 100644
index 000000000000..4395ad64ea97
--- /dev/null
+++ b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/lib/Foo.scala
@@ -0,0 +1,22 @@
+package lib
+
+case class Foo private[lib] (x: Int)
+
+// case object Foo is its own mirror, so the mirror for Foo will be anonymous.
+case object Foo
+
+
+case class Bar protected[lib] (x: Int)
+
+// case object Bar is its own mirror, so the mirror for Bar will be anonymous.
+case object Bar
+
+object OldMirrors {
+  val mFoo = summon[deriving.Mirror.ProductOf[Foo]]
+  val mFooObj = summon[deriving.Mirror.ProductOf[Foo.type]]
+
+  object SubBar extends Bar(1) {
+    val mBar = summon[deriving.Mirror.ProductOf[Bar]]
+    val mBarObj = summon[deriving.Mirror.ProductOf[Bar.type]]
+  }
+}
diff --git a/sbt-test/scala3-compat/anon-prod-mirrors-3.1/project/DottyInjectedPlugin.scala b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/project/DottyInjectedPlugin.scala
new file mode 100644
index 000000000000..fb946c4b8c61
--- /dev/null
+++ b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/project/DottyInjectedPlugin.scala
@@ -0,0 +1,11 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion")
+  )
+}
diff --git a/sbt-test/scala3-compat/anon-prod-mirrors-3.1/test b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/test
new file mode 100644
index 000000000000..63092ffa4a03
--- /dev/null
+++ b/sbt-test/scala3-compat/anon-prod-mirrors-3.1/test
@@ -0,0 +1 @@
+> app/run