Add test without explicit-nulls

Author: noti0na1

tests/pos/i13197.scala

@@ -0,0 +1,12 @@
+// this test is similar to explicit-nulls/pos/i13197.scala, but without explicit nulls
+
+extension [T](x: T | String) inline def forceString: x.type & String =
+  x.asInstanceOf
+
+trait Bar:
+  def b: String | Int
+
+class Foo(a: String = "", b: Any)
+
+object Foo:
+  def foo(bar: Bar) = Foo(b = bar.b.forceString)