Aha! I thought you only captured self because you needed to call a function on it. Your example led me to believe that your function was really pure, and that you were only keeping it as an instance func to avoid having long inline closures.
I certainly agree that it’s useful to be able to reference a function on self, without having to capture it strongly.
I wrote some loose thoughts on this three years ago: How to avoid implicit retain cycles when using function references - sveinhal on swift
About two thirds down you can see an unown function you can create for a few arities