Support Kotlin parameter default values in handler methods

[diesieben07]

Allows using default parameter values for any method parameters resolved
via AbstractNamedValueMethodArgumentResolver, similar to defaultValue
parameter of e.g. @RequestParam annotation, but skipping the converter
chain.

Issues: SPR-16598.

[diesieben07]

Rebased onto current master and resolved merge conflicts.

[raderio]

An ETA when it can be merged?

[raderio]

is something wrong with this pull request?

[snicoll]

@raderio please be patient, we'll get to it as soon as we can.

[sdeleuze]

Current Kotlin API forces us to have a rather involved implementation which is not a good fit with our infra. We would need a reflection API that allows Spring to know an optional parameter default value without calling the function. I will discuss with Kotlin team to see what is possible here in Kotlin 1.3.

[diesieben07]

I am not sure if what you want is possible.
You can write something like this:
fun foo(a: String, b: String = a) or, even worse:
fun foo(a: String, b: String = someFunctionWithSideEffects())

This is impossible to represent in the way you propose.