When a select's value is removed because it's no longer available, recalculation is not triggered

[lognaturel]

Software versions

JavaRosa v3.3.0 and prior

Problem description

ItemsetBinding.getChoices() may mutate a select's answer if the previously-selected choice is no longer available:

javarosa/src/main/java/org/javarosa/core/model/ItemsetBinding.java

Line 83 in c7a85c0

* SIDE EFFECT: any item currently selected as part of the current question's answer that is no longer

This does not trigger recomputation of dependent values or fire the xforms-value-changed event.

Steps to reproduce the problem

    @Test
    public void removingChoiceFromOptions_triggersRecalculation() throws IOException {
        Scenario scenario = Scenario.init("Select", html(
            head(
                title("Select"),
                model(
                    mainInstance(
                        t("data id='select'",
                            t("filter"),
                            t("select"),
                            t("dependent"))),

                    instance("choices",
                        item("aa", "A"),
                        item("aaa", "AA"),
                        item("bb", "B"),
                        item("bbb", "BB")),
                    bind("/data/dependent").calculate("/data/select"))),
            body(
                input("/data/filter"),
                select1Dynamic("/data/select", "instance('choices')/root/item[starts-with(value,/data/filter)]")
            )));

        scenario.answer("/data/filter", "a");
        scenario.answer("/data/select", "aa");
        assertThat(scenario.answerOf("/data/dependent"), is(stringAnswer("aa")));

        scenario.answer("/data/filter", "q");
        assertThat(scenario.choicesOf("/data/select").isEmpty(), is(true));
        assertThat(scenario.answerOf("/data/select"), nullValue());
        assertThat(scenario.answerOf("/data/dependent"), nullValue());
    }

Expected behavior

Recomputation should be triggered as well as the xforms-value-changed event.

This all is making me wonder about the clearing behavior. It's clearly desired in a conventional cascading select context. There are cases like selects without duplicates (see forum post) where it could be surprising. I believe this has been the behavior forever but that might be worth double checking.

See related tests in SelectOneChoiceFilterTest and DynamicSelectUpdateTest.