I'm not sure what you mean here. This fails with a...

# test

[nhaarman]

I'm not sure what you mean here. This fails with a kotlin.NotImplementedError, unless you make the function `open`:

@Test
fun foo() {
    val foo = mock<Foo>()
    foo.foo()
}

open class Foo {

    fun foo() {
        TODO()
    }
}

Whether or not it returns a value doesn't matter here.