Renders of a variable bound to an input element of type="number" will not update if the variable is changed within a reactive statement that is triggered by the input

[EmNudge]

Describe the bug
The title is a hell of a mouthful.
I'm still not certain I've actually pinpointed the odd behavior, but this is as close as I could get.
To elaborate on the title, if an <input> element with type="number" triggers a reactive statement via a bound variable, changing the bound variable in the reactive statement will not perform any updates to places which rely on that variable.

That means, in the following example the reactive statement will trigger when myNum changes (due to the console.log). However, if it was triggered by typing in the <input> element, renders using myNum will be unaffected. If other variables are changed, those will work fine, but ones with myNum will be ignored.

Triggering the reactive statement some other way (like having a button increment myNum) will result in myNum changing to 500.

<script>
  let myNum = 0;
  $: {
    console.log(myNum);
    myNum = 500;
  }
</script>

<input type="number" bind:value={myNum} />
<button on:click={() => myNum++}>change num</button>

As an aside, @pngwn found a fix (not for the bug, but to get this REPL working). Using tick before myNum = 500 allows an update cycle and it should work as it does with type="text":
https://svelte.dev/repl/dca7070e064f4d9da9f49d4133056a76?version=3.20.1

To Reproduce
The previous example in a REPL:
https://svelte.dev/repl/2fb57570c5dd48a580619d13246a53e1?version=3.20.1
Change the type from "number" to "text" to see how its behavior changes.

For an actual use-case (and for how I ran into this), this REPL on restrictive/controlled inputs may help:
https://svelte.dev/repl/8019ec1168fe4fb99e2a2cd44545f70b?version=3.20.1

Expected behavior
like type="text", type="number" should allow myNum's change in value to affect renders.

Information about your Svelte project:

• Your browser and the version: (e.x. Chrome 52.1, Firefox 48.0, IE 10)

• Windows & OSX, REPL, Rollup, Chrome 80, Svelte v3.2

Severity
Medium. Creating restrictive user inputs is useful. With React it's the default way to make inputs. With svelte it's a bit more difficult and may require a bit of twisting.

[Conduitry]

Possibly related to #2816 and #3470.

The difference in behavior between number and text is presumably related to the input_updating guard in the code generated for number but I haven't dug into it any more deeply than that.

[pushkine]

this.needs_lock = name === 'input' && type === 'number'; // TODO others?

line reference
it was introduced as a fix for #3426

[Conduitry]

Fixed in 3.21.0 - https://svelte.dev/repl/2fb57570c5dd48a580619d13246a53e1?version=3.21.0