How to enable Kotlins "unit conversions on arbitrary expressions" - kotlin

Question

I'm getting a Kotlin Error:
The feature "unit conversions on arbitrary expressions" is
experimental and should be enabled explicitly. You can also change the
original type of this expression to (...) -> Unit
My code is the following:
val foo: () -> String = { "Test" }
fun bar(doSometing: () -> Unit) { /* */ }
val baz = bar(foo) // here foo throws the error
It's pretty obvious what i'm doing wrong: bar expects () -> Unit, but i provide () -> String.
However, the error message implies i can opt in to "unit conversions on arbitrary expressions". How do i do this?
The only thing related i could find here on SO doesn't answer my question: https://stackoverflow.com/questions/72238339/kotlin-the-feature-unit-conversion-is-disabled

Answer 1

It's interesting that you can pass a function reference but not an equivalent expression:
fun f(): String = "Test"
val foo = ::f
fun bar(doSometing: () -> Unit) { /* */ }
val baz = bar(::f) // OK
val baz2 = bar(foo) // Error
You can make it compile using the command-line option
-XXLanguage:+UnitConversionsOnArbitraryExpressions but it's not recommended:
$ cat t.kt
val foo: () -> String = { "Test" }
fun bar(doSometing: () -> Unit) { /* */ }
val baz = bar(foo)
$ kotlinc t.kt
t.kt:5:15: error: the feature "unit conversions on arbitrary expressions" is experimental and should be enabled explicitly. You can also change the original type of this expression to (...) -> Unit
val baz = bar(foo)
^
$ kotlinc -XXLanguage:+UnitConversionsOnArbitraryExpressions t.kt
warning: ATTENTION!
This build uses unsafe internal compiler arguments:
-XXLanguage:+UnitConversionsOnArbitraryExpressions
This mode is not recommended for production use,
as no stability/compatibility guarantees are given on
compiler or generated code. Use it at your own risk!
t.kt:3:9: warning: parameter 'doSometing' is never used
fun bar(doSometing: () -> Unit) { /* */ }
^
$

Answer 2

This is because the expected type of doSometing is () -> Unit, but given () -> String, you can fix it by modify return type of foo or modify type of doSometing
both
val foo: () -> Unit = { println("Test") }
fun bar(doSometing: () -> Unit) { doSometing() }
val baz = bar(foo)
.
val foo: () -> String = { "Test" }
fun bar(doSometing: () -> String) { /* */ }
val baz = bar(foo)
works fine.