fix

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

@@ -27,5 +27,4 @@ export const LEGACY_PROPS = Symbol('legacy props');
 export const TEARDOWN_PROPS = Symbol('teardown props');
 export const LOADING_ATTR_SYMBOL = Symbol('');
 
-export const CTX_CONTAINS_TEARDOWN = 1 << 1;
-export const CTX_DESTROYED = 1 << 2;
+export const CTX_DESTROYED = 1 << 1;

packages/svelte/src/internal/client/reactivity/props.js

@@ -5,7 +5,8 @@ import {
 	PROPS_IS_IMMUTABLE,
 	PROPS_IS_LAZY_INITIAL,
 	PROPS_IS_RUNES,
-	PROPS_IS_UPDATED
+	PROPS_IS_UPDATED,
+	UNINITIALIZED
 } from '../../../constants.js';
 import { get_descriptor, is_function } from '../../shared/utils.js';
 import { mutable_source, set, source, update } from './sources.js';
@@ -23,6 +24,7 @@ import {
 import { proxy } from '../proxy.js';
 import { capture_store_binding } from './store.js';
 import { legacy_mode_flag } from '../../flags/index.js';
+import { is_runes } from '../context.js';
 
 /**
  * @param {((value?: number) => number)} fn
@@ -391,6 +393,11 @@ export function prop(props, key, flags, fallback) {
 		return (inner_current_value.v = parent_value);
 	});
 
+	// Read the prop eagerly to ensure it has a value
+	if (!is_runes()) {
+		get(current_value);
+	}
+
 	if (!immutable) current_value.equals = safe_equals;
 
 	return function (/** @type {any} */ value, /** @type {boolean} */ mutation) {

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

@@ -22,8 +22,7 @@ import {
 	ROOT_EFFECT,
 	LEGACY_DERIVED_PROP,
 	DISCONNECTED,
-	BOUNDARY_EFFECT,
-	CTX_CONTAINS_TEARDOWN
+	BOUNDARY_EFFECT
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set } from './reactivity/sources.js';
@@ -567,17 +566,7 @@ export function update_effect(effect) {
 
 		execute_effect_teardown(effect);
 		var teardown = update_reaction(effect);
-
-		if (typeof teardown === 'function') {
-			var ctx = effect.ctx;
-			if (ctx !== null && (ctx.f & CTX_CONTAINS_TEARDOWN) === 0) {
-				ctx.f ^= CTX_CONTAINS_TEARDOWN;
-			}
-			effect.teardown = teardown;
-		} else {
-			effect.teardown = null;
-		}
-
+		effect.teardown = typeof teardown === 'function' ? teardown : null;
 		effect.wv = write_version;
 
 		var deps = effect.deps;

packages/svelte/tests/runtime-legacy/samples/ondestroy-prop-access/Component.svelte

@@ -1,11 +1,14 @@
 <script>
-	import { onDestroy } from 'svelte';
+	import { onDestroy } from "svelte";
 
-	export let my_prop;
+	export let checked;
+	export let count;
 
 	onDestroy(() => {
-		console.log(my_prop.foo);
+		console.log(count, checked);
 	});
 </script>
 
-{my_prop.foo}
+<p>{count}</p>
+
+<button onclick={()=> count-- }></button>