Can anybody explain this compiler behavior?

class TestNullableProblem {

    var foo: String? = "a"
    var bar: String? = null

    @Test fun `does not compile`() {
        foo = null
        // Fails to compile due to overload resolution ambiguity.
        assertSomething(foo)
    }

    @Test fun `compiles 1`() {
        // Why does this compile? bar is definitely null, so we'd expect the same overload ambiguity error as before.
        assertSomething(bar)
    }

    @Test fun `compiles 2`() {
        foo = "b"
        assertSomething(foo)
    }

    @Test fun `compiles 3`() {
        assertSomething(foo)
    }

    private fun assertSomething(v: String?): Unit = TODO()
    private fun assertSomething(v: Int?): Unit = TODO()
}