Kotlin null and kotlin.Unit confusion in expression body

Question

Could anyone explain the result of following code snippet? Why the first function call returns kotlin.Unit?

kotlin version 1.5.21, java11

fun <T> testMethod(data: String?): T? = data?.let { return null }
println(testMethod<String>(null))  // --> kotlin.Unit

fun <T> testMethod2(data: String?): T? = data?.let { null }
println(testMethod2<String>(null)) // --> null

Why the result is different in kotlin 1.4.30 - https://pl.kotl.in/uFZDE3O9e?

Answer 1

It was a compiler issue and already reported here - https://youtrack.jetbrains.com/issue/KT-47527, fixed in 1.6.0-dev-970.

Ref: https://github.com/jetbrains/kotlin/commit/ae608ea67fc589c4472657dc0317e97cb67dd158