Merge pull request #4913 from dotty-staging/fix-#3500

Fix #3500: Add regression test

Author: smarter

tests/pos/i3500.scala

@@ -0,0 +1,10 @@
+trait Map2[K] {
+  def get(k: K): K = k
+  def foo: K = {
+    this match {
+      case that: Map2[b] => that.get(3.asInstanceOf[b])
+      case that: Map2[b] => that.get(3.asInstanceOf[K])
+      case _ => get(5.asInstanceOf[K])
+    }
+  }
+}