fix: always reconnect deriveds in get, when appropriate (#17451)

* failing test

* fix: always reconnect deriveds in get, when appropriate

* shuffle code around a bit

* note to self

.changeset/major-towns-grab.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: always reconnect deriveds in get, when appropriate

packages/svelte/src/internal/client/runtime.js

@@ -596,14 +596,14 @@ export function get(signal) {
 		}
 	}
 
-	if (is_destroying_effect) {
-		if (old_values.has(signal)) {
-			return old_values.get(signal);
-		}
+	if (is_destroying_effect && old_values.has(signal)) {
+		return old_values.get(signal);
+	}
 
-		if (is_derived) {
-			var derived = /** @type {Derived} */ (signal);
+	if (is_derived) {
+		var derived = /** @type {Derived} */ (signal);
 
+		if (is_destroying_effect) {
 			var value = derived.v;
 
 			// if the derived is dirty and has reactions, or depends on the values that just changed, re-execute
@@ -619,14 +619,13 @@ export function get(signal) {
 
 			return value;
 		}
-	} else if (
-		is_derived &&
-		(!batch_values?.has(signal) || (current_batch?.is_fork && !effect_tracking()))
-	) {
-		derived = /** @type {Derived} */ (signal);
 
-		if (is_dirty(derived)) {
-			update_derived(derived);
+		// TODO this should probably just be `!batch_values?.has(derived)` — the second bit
+		// should be taken care of by clearing `batch_values` in `mark_reactions`?
+		if (!batch_values?.has(derived) || (current_batch?.is_fork && !effect_tracking())) {
+			if (is_dirty(derived)) {
+				update_derived(derived);
+			}
 		}
 
 		if (is_updating_effect && effect_tracking() && (derived.f & CONNECTED) === 0) {

packages/svelte/tests/runtime-runes/samples/async-fork-snippet-dev/_config.js

@@ -0,0 +1,24 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	async test({ assert, target }) {
+		const [fork] = target.querySelectorAll('button');
+
+		fork.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>fork</button><button>false</button>');
+
+		const [, toggle] = target.querySelectorAll('button');
+
+		toggle.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>fork</button><button>true</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-snippet-dev/main.svelte

@@ -0,0 +1,26 @@
+<script>
+	import { fork } from 'svelte';
+
+	let condition = $state(false);
+	let checked = $state(false);
+
+	const d = $derived({ checked });
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		condition = true;
+	}).commit();
+}}>fork</button>
+
+{#if condition}
+	<!-- in dev, snippet arguments are read eagerly, outside a tracking context -->
+	<!-- this test checks that doing so doesn't prevent the derived from connecting -->
+	{#snippet foo({ checked })}
+		{checked}
+	{/snippet}
+
+	<button onclick={() => (checked = !checked)}>
+		{@render foo(d)}
+	</button>
+{/if}