Add regresion test for scala#21576

tests/run/i21576.check

@@ -0,0 +1,3 @@
+public SubFlow(SubFlowDef<Flow1<In>>,SubFlowDef<Flow2<java.lang.Object>>)
+public SubFlowDef<Flow1<In>> SubFlow.delegate1()
+public SubFlowDef<Flow2<java.lang.Object>> SubFlow.delegate2()

tests/run/i21576.scala

@@ -0,0 +1,31 @@
+// scalajs: --skip
+import scala.annotation.unchecked.uncheckedVariance
+
+trait SubFlowDef[+F[+_]]
+final class Flow1[-In]{
+  type Repr[+O] = Flow1[In @uncheckedVariance]
+}
+final class Flow2[+Out]{
+  type Repr[+O] = Flow2[O]
+}
+class SubFlow[In, Out](
+    val delegate1: SubFlowDef[Flow1[In]#Repr],
+    val delegate2: SubFlowDef[Flow2[Out]#Repr]
+)
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    classOf[SubFlow[?, ?]]
+      .getConstructors()
+      .map(_.toGenericString())
+      .sorted
+      .foreach(println)
+
+    classOf[SubFlow[?, ?]]
+      .getMethods()
+      .filter(_.getName().startsWith("delegate"))
+      .map(_.toGenericString())
+      .sorted
+      .foreach(println)
+  }
+}