KotlinSAMIssue.kt

// Accoriding Kotlin 1.4.0 release blog, we should be able to define SAM interfaces in Kotlin and use them the
// same way we can use them if they are defined in Java but the code below still does not compile
// https://stackoverflow.blog/2020/08/26/kotlin-1-4-released-to-improve-performance/


// As typealias (works)
typealias FooTypeAlias = () -> Boolean
fun runAsTypeAlias(func: FooTypeAlias): Boolean {
    return func.invoke()
}

// As Java SAM interface (works)
// Defined in FooJavaInterface.java: public interface FooJavaInterface { boolean samJavaInterfaceCall();}
fun runAsJavaInterface(func: FooJavaInterface): Boolean {
    return func.samJavaInterfaceCall()
}

// As Kotlin SAM interface (doesn't work)
interface FooKotlinInterface {
    fun samKotlinInterfaceCall(): Boolean
}
fun runAsKotlinInterface(func: FooKotlinInterface): Boolean {
    return func.samKotlinInterfaceCall()
}

// Function that *should* work for typealias, Java SAM interface, and Kotlin SAM interface calls
fun doSomething(): Boolean {
    return true
}

fun main() {
    // prints 1.4.0
    println(KotlinVersion.CURRENT)

    // compiles
    runAsTypeAlias { doSomething() }

    // compiles
    runAsJavaInterface { doSomething() }

    // Should compile in kotlin 1.4.0 but doesn't:
    //      Type mismatch.
    //      Required:
    //      FooKotlinInterface
    //      Found:
    //      () → Boolean
    runAsKotlinInterface { doSomething() }
}

Hello @pbriggs28,

as written under the SO blog post you need to declare the Kotlin SAM interface as fun interface FooKotlinInterface instead of interface FooKotlinInterface, see the official Kotlin docs for more information