As discussed in the PR, current Kotlin API force us to integrate that as a rather involved and not very elegant way, I will raise and discuss that point with Kotlin team. As a consequence, I think it is more reasonable to target 5.2 for that one.
Resolving the value in Spring (like a defaultValue) is not feasible, because Kotlin's default is not a simple value but a piece of essentially arbitrary object-internal kotlin code, which Spring has no access to.
An alternative approach would be to use @JvmOverloads to have kotlin generate explicit permutations of the method signature. Spring would need to select the right one depending on which params are present, in stead of rejecting them all as ambiguous endpoint mappings like it does now. One would still need to define a sane course of action if that annotation is missing. Bad request?
In any case: the situation today is that a kotlin endpoint with a default value will run without incident right up until a request without the parameter arrives, in which case it will just throw an NPE from way inside Spring's parameter mapping. That has got to be the wrong approach whichever way you look at it.