Add regression test for #2941

Author: griggt

tests/pos/i2941.scala

@@ -0,0 +1,17 @@
+trait FooBase {
+  type Bar >: Null <: BarBase { type This <: FooBase.this.Bar }
+  type This >: this.type <: FooBase { type This <: FooBase.this.This }
+
+  def derived(bar: Bar): This = ???
+}
+
+trait BarBase {
+  type This >: Null <: BarBase { type This <: BarBase.this.This }
+}
+
+object Test {
+  def bad(foo: FooBase): FooBase = foo match {
+    case foo: FooBase =>
+      foo.derived(???)  // Triggers infinite loop in TypeAssigner.avoid()
+  }
+}