https://github.com/lampepfl/dotty/pull/8170 https://github.com/lampepfl/dotty/pull/8212 - be aware of an optional third type tree in a type bounds type tree, which is the alias of an opaque type https://github.com/lampepfl/dotty/pull/8428
