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:thread: hey guys, I’m facing an issue with Kotlin type system and generic class types. In particular, I’m working with Akka Typed and its `ReceiveBuilder#onMessage` signature.

The problem is that the behavior itself is generic:

`private class MyBehavior&lt;T&gt; : AbstractBehavior&lt;Message&lt;T&gt;&gt;`

the receive builder then has a method with the signature:

`onMessage(type: Class&lt;T&gt;, handler: Function&lt;T, Behavior&lt;T&gt;&gt;)`

`type` is supposed to be a subtype of `T`, but with the generics argument in there I can’t get it to compile without an explicit cast

is that the only way, or is there a better solution?

CleanShot 2021-11-19 at 11.03.05.png

does this work?  `fun &lt;T, Y: Class&lt;T&gt;&gt; onMessage(type: Y, handler: Function&lt;T, Behavior&lt;T&gt;&gt;)`

CleanShot 2021-11-19 at 11.03.41.png

without the cast I’ll get a type mismatch

<@UVAR0DXP0> not really; the problem is that `::class` will always get the star projection, not the generic variant I need.

in Java I can do `.onMessage(SubType.class, this::handler),` where handler is `private Behavior&lt;...&gt; handler(SubType&lt;T&gt; msg)`

I get an unchecked generics warning, but it does work; apparently in Kotlin the best I can do is creating that var with the class object

I tried defining an extension method with a reified type param, but it’s also quite verbose — the compiler doesn’t infer the right types from the handler alone.

that is:
```inline fun &lt;T, reified U : T&gt; ReceiveBuilder&lt;T&gt;.onMessage(noinline handler: (U) -&gt; Behavior&lt;T&gt;): ReceiveBuilder&lt;T&gt; {
    return onMessage(U::class.java, handler)
}```


and then I can do:
```        return newReceiveBuilder()
            .onMessage&lt;MessageProcessorDriverMessage&lt;T&gt;, MessageProcessorDriverMessageReceived&lt;T&gt;&gt; { onMessage(it) }```


wait, that does work well if I use a method reference!

after defining…
```inline fun &lt;T, reified U : T&gt; ReceiveBuilder&lt;T&gt;.addHandler(noinline handler: (U) -&gt; Behavior&lt;T&gt;): ReceiveBuilder&lt;T&gt; {
    return onMessage(U::class.java, handler)
}```


I can do
```return new ReceiveBuilder().addHandler(::onMessage)```


great work <@U02GBMMEG5R>! :slightly_smiling_face: