test: add in a regression test for #14271 (#17509)

[skip community_build]

closes #14271

Author: nicolasstucki

tests/pos/i14271.scala

@@ -0,0 +1,14 @@
+// https://github.com/lampepfl/dotty/issues/14271
+class Bound[T]
+class MyClass[T <: Bound[T]]
+
+class Container[V] {
+  def doSth(): V = ???
+}
+
+def bug() = {
+  val m = new Container[MyClass[_]]
+  if (true) {
+    m.doSth()
+  }
+}