Component two-way binding edge case

[Rich-Harris]

This might be a wontfix (it's a niche problem, and it's very easily worked around) but I don't have time to fully investigate it right now, so just leaving myself some breadcrumbs:

<p>y: {{y}}</p>

{{#if x}}
  <Foo bind:y/>
{{else}}
  <Bar bind:y/>
{{/if}}

<Baz bind:x/>

<script>
  import Foo from './Foo.html';
  import Bar from './Bar.html';
  import Baz from './Baz.html';

  export default {
    components: {
      Foo,
      Bar,
      Baz
    }
  };
</script>

// Foo.html
<p>y: {{y}}</p>

<script>
  export default {
    data: () => ({
      y: 'foo'
    })
  };
</script>

// Bar.html
<p>y: {{y}}</p>

<script>
  export default {
    data: () => ({
      y: 'bar'
    })
  };
</script>

// Baz.html
<script>
  export default {
    data: () => ({
      x: true
    })
  };
</script>

That looks like it should result in this...

<p>y: foo</p>
<p>y: foo</p>

...but in fact it ends up like this:

<p>y: undefined</p>
<p>y: foo</p>

That's because on the initial pass, the {{else}} part is rendered, which initialises the binding between Main and Bar (because x doesn't yet have a value). Then, Svelte encounters the <Baz bind:x> component, and creates a new binding.

Bindings are flushed, which means that x has a value, which means that Bar is torn down (which has the effect of setting its _state to {}). Then the Bar binding is triggered, and y is set to undefined in Main.

As a starting point, no observer should be created when triggering a binding on a torn down component. (Perhaps observing a torndown component should also have no effect, or even throw — possibly a separate question.)

Changing the order of things in the template in Main.html yields different results:

<p>y: {{y}}</p>

<Baz bind:x/>

{{#if x}}
  <Foo bind:y/>
{{else}}
  <Bar bind:y/>
{{/if}}

<p>y: bar</p>
<p>y: bar</p>

(bar and not foo because by the time <Foo bind:y/> is created, y has a value in the parent component, and the child won't override the parent. Removing the phantom observer would presumably fix that as well.)