Why is there a platform clash for generic suspend lambda and regular Flow lambda

I want to create a function with two overloads:

• One that takes a suspend function that takes no arguments and returns Any
• One that takes a regular function that takes one argument: Any and returns Any

This is the code I tried to write:

import kotlinx.coroutines.flow.Flow

fun test(handler: suspend () -> Any) { handler.toString() }
fun test(handler: (Any) -> Flow<Any>) { handler.toString() }

And this is the error i got:

> Platform declaration clash: The following declarations have the same
> JVM signature (test(Lkotlin/jvm/functions/Function1;)V):
> fun test(handler: (Any) -> Flow<Any>): Unit defined in se.vermiculus.vericlear.webserver.utility
> fun test(handler: suspend () -> Any): Unit defined in se.vermiculus.vericlear.webserver.utility

The reason is that suspending lambda types are represented in the JVM as having an extra Continuation parameter. See also the example presented here, or just try to call your Kotlin function from a Java file in IntelliJ.

What happens under the hood when you call the suspending lambda is, all the code after the call gets wrapped up in a "continuation", and also gets passed to the lambda. That's the code that will "continue" to run after the suspension is done.

Because of the extra parameter in the suspend lambda, both of your functions have 1 parameter in the JVM representation, and hence are both represented as Function1s, hence the conflict.

In Java, suspend () -> Any would become

Function1<? super Continuation<? super Object>, ? extends Object> 

and (Any) -> Flow<Any> becomes

Function1<? super Object, ? extends Flow<? extends Object>>

Though it's not like that's any useful for solving the conflict I recommend just sticking some @JvmNames on it.

This is because both functions are translated into the same bytecode.

Something like this:

import kotlin.jvm.functions.Function1;
import kotlin.jvm.internal.Intrinsics;
import org.jetbrains.annotations.NotNull;

public final void test(@NotNull Function1 handler) {
    Intrinsics.checkNotNullParameter(handler, "handler");
    handler.toString();
}