One-line function call on optional value yields Kotlin compile error: Type mismatch: inferred type is Unit? but Unit was expected

If a Void function has a single-line that invokes another Void function, we transpile it into a single line like: fun x(): Unit = f(), but that raises a compile error because f() returns Unit?.

For example, the Swift:

class Thing {
    var t: Thing? = nil

    init() {
    }

    func go() {
    }

    func optionalCall() {
        t?.go()
    }
}

transpiles into the Kotlin:

internal open class Thing {
    internal open var t: Thing? = null

    internal constructor() {
    }

    internal open fun go() = Unit

    internal open fun optionalCall(): Unit = t?.go()
}

which yields the compile error:

Type mismatch: inferred type is Unit? but Unit was expected

The only workaround is to add another line to the function, like:

    func optionalCall() {
        let x = t?.go()
        _ = x
    }

Start from the transpilation path for Swift Void functions whose single statement is an optional call, using the Thing example to reproduce the generated Kotlin and compiler error. Verify the fix by checking that optionalCall produces Kotlin accepted by the compiler without requiring an extra statement.