Fix #19633: Add regression test. (#20013)

Not sure what fixed it, but it seems to be good now.

Author: sjrd

tests/pos/i19633.scala

@@ -0,0 +1,24 @@
+object Repro {
+  trait Responsive {
+    type Response
+  }
+
+  object Responsive {
+    type Aux[R] = Responsive {type Response = R}
+    type Response[R] = R match {case Aux[r] => r}
+  }
+
+  case class StringRequest(name: String) extends Responsive {
+    type Response = String
+  }
+
+  def withImplicit[R <: Responsive](request: R)(implicit ct: scala.reflect.ClassTag[Responsive.Response[R]]): Responsive.Response[R] = ???
+
+  def withFunction[R <: Responsive](request: R)(call: R => Responsive.Response[R]): Responsive.Response[R] = ???
+
+  def stringWithFunction(req: StringRequest): String = withFunction(req)(_.getClass.getSimpleName)
+
+  def stringWithImplicit(req: StringRequest): String = withImplicit(req)
+
+  def main(args: Array[String]): Unit = {}
+}