Are there any other languages where typing is purely optional and yet so much effort is going into developing the typing system?

I started experimenting with Kotlin just when Python was starting to get type hints and after my experience with Kotlin I am totally sold on the benefits of typing. So to me it's great to see these recent developments. It just feels weird to me to have a language that is fundamentally dynamically typed (and committed to that paradigm) invest so much effort in developing this elaborate but optional typing system.