Error in destructuring let on non-let content update

[simonbuchan]

See repl example

When externally updating the content of a component with a let binding (e.g. not due to a change of the let binding), and the binding has a destructuring expression, the destructuring expression is evaluated with undefined, which will cause an error.

In the example, note that the first <Await/> block does not destructure, and works correctly when the Increment button is pressed, while the second third, otherwise identical except for the destructuring, causes the error when the Increment button is pressed:

VM516:738 Uncaught (in promise) TypeError: undefined is not iterable (cannot read property Symbol(Symbol.iterator))
    at Array.Await.props.$$slots.default (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:738:36)
    at get_slot_changes (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:63:64)
    at Object.update [as p] (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:470:21)
    at Object.update [as p] (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:558:17)
    at update (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:173:16)
    at flush (eval at handle_message (VM365 about:srcdoc:68), <anonymous>:143:5)

[fschuetz04]

I also came across this issue while working on a drag and drop component. I created a simpler example here; https://svelte.dev/repl/cf7c98699d5a495b96945e42a63ffeed?version=3.12.1.

[Conduitry]

Fixed (finally!) in 3.16.6.