Why configurations.all { conf: Configuration -> ... } compile error with: Type mismatch: inferred type is Unit but Boolean was expected

Question

I use build.gradle.kts to build my android project. The following code does NOT compile:

configurations.all { conf: Configuration ->
    println(conf)
}

error is:

println(conf)
^ Type mismatch: inferred type is Unit but Boolean was expected

But if you remove the conf:Configuration -> declaration, it compiled:

configurations.all {
    println(conf)
}

I know that the first one matches Iteralbe.all which need a boolean return value, while the second matches DomainObjectCollection.all. But why?

Thanks if any answer

Answer 1

Finally I found the answer here: Kotlin not able to convert gradle's Action class to a lambda.

here: https://discuss.kotlinlang.org/t/how-and-why-is-the-annotation-hasimplicitreceiver-from-the-gradle-api-working/11231

and the official doc: https://kotlinlang.org/docs/reference/compiler-plugins.html?_ga=2.154315515.2101643340.1565659650-1852326073.1562576288#sam-with-receiver-compiler-plugin