There are several bugs reported already regarding "illegal cyclic reference", the closest one probably being #7642. What might be special about this one is that it involves a type member of a constructor argument (foo.A below). What seems to confuse the type checker is the fact that the constructor argument also overrides a val.

trait Foo {
  type A
}

trait Bar {
  type B
  val foo: Foo { type A = B }
}

class Bar1(val foo: Foo) extends Bar {
  type B = foo.A // error: illegal cyclic reference involving type B
}

// workaround
class Bar2(val foo0: Foo) extends Bar {
  type B = foo0.A // OK
  val foo = foo0  // OK
}