> Really? Perhaps read the 'Principle of Least Astonishment' [1] to see why this is a problem. If I create a new object I would expect the 'init time' properties to be initialised.

The Principle of Least Astonishment definitely applies. It's a deliberate design choice that unfortunately violates the principle.

> Great insight. Let's not run the 'init time' properties for a newly initialised object, just in case it works as expected. This 'feature' can't even be manually resolved by doing post-`with` updates (because often the properties are init/read-only). It makes the whole init-property feature brittle as fuck.

? I'm not sure I follow what you are going with here, but yeah, in general you'd have to carefully limit all uses of "with" for objects with precomputed values to inside the encapsulation of said objects. Alternatively, as you mentioned, you could just not have precomputed properties.