Additionally, is there any way to know if `TypeResolutionInterceptorExtension` will become non-inter...

# compiler

[FalseHonesty]

Additionally, is there any way to know if TypeResolutionInterceptorExtension will become non-internal anytime soon? I'd like to be able to use it in my compiler plugin 🙂

[Roman Artemev [JB]]

No, in contrary we are going to remove it eventually. Could you please explain what is your case?

[FalseHonesty]

Sure, no problem. I have a

abstract class Parent {
    fun action() = apply {
        // do action
    }
}

which then gets extended by a bunch of children, like

class Child : Parent() {
}

[FalseHonesty]

and I want child.action() to return the instance of Child

[FalseHonesty]

Because I know that in the library my plugin is for, that function will always return the type that it was called on

[FalseHonesty]

Thinking about it, there would have to be an IrGenerationExtension to create the cast operation, but that wouldn't suffice would it? Wouldn't the type system still not realize it would be of the new type Child?

[Roman Artemev [JB]]

I would suggest something like

abstract class Parent<T: Parent> {
    open fun action(): T = apply {
        // do action
    }
}

class Child : Parent<Child>() {
 // override action(): Child
}

So in that case you could always inject override member fun action(): Child which will be properly compiled into what you exactly need including bridge method.

[FalseHonesty]

Hmm, that doesn't compile though, it says the reference to Parent in the bound for T needs a type argument

[FalseHonesty]

And it also would mean that all references to Parent would now need to have a type specified, even if that is just *

[Roman Artemev [JB]]

but that wouldn’t suffice would it

If you goal is to make sure that types are correct it should to be done automatically since compiler inserts “implicit cast” wherever types are mismatch.

[FalseHonesty]

Oh interesting, but how, if possible, could I make it so the compiler knows the returned object is of the same type the function was called on?

[Roman Artemev [JB]]

Ok so am I correct that you need to keep override-chain. I mean action in Child has to override it from Parent?

[Roman Artemev [JB]]

Oh, I see

[FalseHonesty]

So, preferably I wouldn't override anything in Child, rather, whenever i had an instance of Child and called any method from Parent on it, it would return the type Child instead of Parent, because I know that in my library it will always return the exact same instance that it was called with

[Roman Artemev [JB]]

So why I just don’t insert?

class Child : Parent() {
  override fun action(): Child { .. }
}

[Roman Artemev [JB]]

And yes, you could transform your code using IrGenerationExtension if you just add cast like this

use1(p.action()) -> use1(cast<Parent>(p.action()))
use2(c.action()) -> use2(cast<Child>(c.action()))

[FalseHonesty]

Oh, like synthetically insert those? I guess that is possible, but a little inconvenient, because Parent has a load of methods that would need overriding. Additionally, Parent and Child are classes in a library, and it was suggested in the 1.4 release notes that libraries shouldn't compile with IR quite yet, but I guess I can require that the library use the IR compiler

[FalseHonesty]

what do you mean by use1 and that code block? I'm slightly confused to the context where that would be used

[FalseHonesty]

The optimal situation would be if I could simply make call-sites of those parent functions understand it returns the child's type and cast there, but I can probably live with the synthetic override solution

[FalseHonesty]

but in a case like:

val child: Child
child = Parent().action()

the code doesn't even make it to the IrGenerationExtension because it fails type inference, saying that the inferred type is Parent, not Child, so I can't automatically insert a cast, somehow I'd need to tell the compiler the type earlier, and as you said, it might implicitly cast

[Roman Artemev [JB]]

use1/2(..) is just a meta of any possible usage. In our snippet such usage is assign into child

[Roman Artemev [JB]]

So to make long story short as I understood you need something like downcast for invocations of some specific method

[Roman Artemev [JB]]

Override/class body modification is not good since it’s in external library.

[FalseHonesty]

Yes precisely. The method doesn't have to be specific per se, rather, its any method on Parent that returns the type Parent, but I think the general point is the same, yes.

[Roman Artemev [JB]]

I think we need to get level up.
What the reason that there is a need to violate type system? I mean they original goal

[FalseHonesty]

Well, currently a lot of code using the library has assignments that require casts, i.e

val myChild = Child().action() as Child

because later on, code often needs to use functions declared specifically in the child,

myChild.childAction()

which means the myChild variable needs to be of type Child. It's not too bad writing it with casts, but there are also a lot of parent functions that are infix, have lambdas, etc. and sometimes the assignment expression become complex, which means we need to wrap the left hand side of the cast in parentheses, just making the code much worse to read

[Roman Artemev [JB]]

I see, thanks.

And as I understand it’s close to impossible to fix the Parent’s interface adding required methods there.

[FalseHonesty]

Do you mean that there probably isn't a good solution available by purely changing the Parent class? If so, yeah I thought that might be the case, which is why I started looking into making a compiler plugin in the first place

[Roman Artemev [JB]]

Yes, at this moment at least. I agree that plugin api has to be as flexible as possible but at the same time its ability should be in some reasonable bounds 🙂

Your case probably worth to put some thoughts in it.

Using Interceptor API is not safe and we very wish to drop as soon as possible. It was originally “designed” (quotes means that is hack) to capture some additional information from callsite.

[FalseHonesty]

Gotcha, makes sense. I'll try to keep updated on the matter, thanks so much for your help, and I hope that the API eventually will be able to support a feature like this 🙂

[Roman Artemev [JB]]

You are always welcome 🙂

[FalseHonesty]

oh by the way, should I file some kind of youtrack issue for this (with this slack thread for context)?

[Roman Artemev [JB]]

Yeah, it would be great 🙂

[FalseHonesty]

I've opened the issue here, https://youtrack.jetbrains.com/issue/KT-41267, thanks again for your time