fix: visit the derived function when to update the dependencies even when it's reassigned

Author: paoloricciuti

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

@@ -10,17 +10,18 @@ export const DISCONNECTED = 1 << 9;
 export const CLEAN = 1 << 10;
 export const DIRTY = 1 << 11;
 export const MAYBE_DIRTY = 1 << 12;
-export const INERT = 1 << 13;
-export const DESTROYED = 1 << 14;
-export const EFFECT_RAN = 1 << 15;
+export const ASSIGNED_DERIVED = 1 << 13;
+export const INERT = 1 << 14;
+export const DESTROYED = 1 << 15;
+export const EFFECT_RAN = 1 << 16;
 /** 'Transparent' effects do not create a transition boundary */
-export const EFFECT_TRANSPARENT = 1 << 16;
+export const EFFECT_TRANSPARENT = 1 << 17;
 /** Svelte 4 legacy mode props need to be handled with deriveds and be recognized elsewhere, hence the dedicated flag */
-export const LEGACY_DERIVED_PROP = 1 << 17;
-export const INSPECT_EFFECT = 1 << 18;
-export const HEAD_EFFECT = 1 << 19;
-export const EFFECT_HAS_DERIVED = 1 << 20;
-export const EFFECT_IS_UPDATING = 1 << 21;
+export const LEGACY_DERIVED_PROP = 1 << 18;
+export const INSPECT_EFFECT = 1 << 19;
+export const HEAD_EFFECT = 1 << 20;
+export const EFFECT_HAS_DERIVED = 1 << 21;
+export const EFFECT_IS_UPDATING = 1 << 22;
 
 export const STATE_SYMBOL = Symbol('$state');
 export const STATE_SYMBOL_METADATA = Symbol('$state metadata');

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

@@ -29,7 +29,7 @@ import {
 	MAYBE_DIRTY,
 	BLOCK_EFFECT,
 	ROOT_EFFECT,
-	EFFECT_IS_UPDATING
+	ASSIGNED_DERIVED
 } from '../constants.js';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
@@ -172,7 +172,11 @@ export function internal_set(source, value) {
 		}
 
 		if ((source.f & DERIVED) !== 0) {
-			set_signal_status(source, (source.f & UNOWNED) === 0 ? CLEAN : MAYBE_DIRTY);
+			// if we are assigning a derived we set it to clean but we also
+			// keep the assigned derived flag so that we can read the dependencies
+			// in the get function
+			var flags = (source.f & UNOWNED) === 0 ? CLEAN : MAYBE_DIRTY;
+			set_signal_status(source, flags | ASSIGNED_DERIVED);
 		}
 
 		mark_reactions(source, DIRTY);

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

@@ -23,7 +23,8 @@ import {
 	LEGACY_DERIVED_PROP,
 	DISCONNECTED,
 	BOUNDARY_EFFECT,
-	EFFECT_IS_UPDATING
+	EFFECT_IS_UPDATING,
+	ASSIGNED_DERIVED
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
@@ -926,6 +927,11 @@ export function get(signal) {
 
 		if (check_dirtiness(derived)) {
 			update_derived(derived);
+		} else if ((derived.f & ASSIGNED_DERIVED) !== 0) {
+			// if the derived is clean but has been assigned a new value, then we need to
+			// still invoke the derived function and update the dependencies or else
+			// it will not depend on the original source
+			update_reaction(derived);
 		}
 	}
 
@@ -1043,7 +1049,7 @@ export function untrack(fn) {
 	}
 }
 
-const STATUS_MASK = ~(DIRTY | MAYBE_DIRTY | CLEAN);
+const STATUS_MASK = ~(DIRTY | MAYBE_DIRTY | CLEAN | ASSIGNED_DERIVED);
 
 /**
  * @param {Signal} signal

packages/svelte/tests/signals/test.ts

@@ -1089,12 +1089,29 @@ describe('signals', () => {
 			assert.equal($.get(b), 1);
 
 			set(a, 2);
+			assert.equal($.get(b), 2);
 			set(b, 3);
 
 			assert.equal($.get(b), 3);
 		};
 	});
 
+	test("unowned deriveds set after they are DIRTY doesn't get updated on get", () => {
+		return () => {
+			const a = state(0);
+			const b = derived(() => $.get(a));
+			const c = derived(() => $.get(b));
+
+			set(b, 1);
+			assert.equal($.get(c), 1);
+
+			set(a, 2);
+
+			assert.equal($.get(b), 2);
+			assert.equal($.get(c), 2);
+		};
+	});
+
 	test('deriveds containing effects work correctly when used with untrack', () => {
 		return () => {
 			let a = render_effect(() => {});