More useful error message when a const is used as part of a reactive assignment

[Morklympious]

Hello! I've run into an issue I resolved thanks to the discord, but this seems useful for anyone coming into v3 (either from v2 or another framework).

Here's a small component I was experiencing trouble with:

<script>
    export const position = "TR";

    const stances = {
        TL : "10 10",
        TR : "90 10",
        BL : "10 90",
        BR : "90 90",
    };

    $: stance = stances[position];
</script>

<style>
    .svg {
        width: 3rem;
        height: 3rem;
        padding: 1rem;
    }
</style>


<svg class="svg" viewBox="0 0 100 100">
    <g class="group" fill="gray">
        <polygon points="0 50, 50 0, 100 50, 50 100" />
        <path d="{stance}" stroke="black" stroke-width="2"/>
    </g>
</svg>

(This repros in REPL)

The error I get when I try to compile this component is ReferenceError: stance is not defined. As it turns out, the solution is changing the stances object from a const to a let. It wasn't immediately obvious to me that all components of a reactive assignment must be mutable, so I think an error that calls that out will save a lot of headaches.

Something along the lines of "A reactive assignment cannot use variables declared with const".

[Morklympious]

Oops. I'm realizing now that I declared TWO const variables and that's why it was yelling. my want for a better error message still stands, but I can get away with a const map if my position is a let

[EmilTholin]

This was resolved in 3.2.0.

REPL