fix(signals): avoid reading unrelated state slices in patchState

modules/signals/spec/state-source.spec.ts

@@ -3,6 +3,7 @@ import {
   effect,
   EnvironmentInjector,
   Injectable,
+  linkedSignal,
   signal,
 } from '@angular/core';
 import { TestBed } from '@angular/core/testing';
@@ -61,6 +62,19 @@ describe('StateSource', () => {
   });
 
   describe('patchState', () => {
+    it('does not read unrelated state slices for partial state objects', () => {
+      const name = signal('');
+      const inaccessible = linkedSignal(() => {
+        throw new Error('Failed to read state slice');
+      });
+      const stateSource = {
+        [STATE_SOURCE]: { name, inaccessible },
+      };
+
+      expect(() => patchState(stateSource, { name: 'John' })).not.toThrow();
+      expect(name()).toBe('John');
+    });
+
     [
       {
         name: 'with signalState',

modules/signals/src/state-source.ts

@@ -82,13 +82,18 @@ export function patchState<State extends object>(
     Partial<NoInfer<State>> | PartialStateUpdater<NoInfer<State>>
   >
 ): void {
-  const currentState = untracked(() => getState(stateSource));
+  const requiresCurrentState = updaters.some(
+    (updater) => typeof updater === 'function'
+  );
+  const currentState = requiresCurrentState
+    ? untracked(() => getState(stateSource))
+    : undefined;
   const newState = updaters.reduce(
     (nextState: State, updater) => ({
       ...nextState,
       ...(typeof updater === 'function' ? updater(nextState) : updater),
     }),
-    currentState
+    currentState ?? ({} as State)
   );
 
   const signals = stateSource[STATE_SOURCE];
@@ -97,7 +102,10 @@ export function patchState<State extends object>(
   for (const key of Reflect.ownKeys(newState)) {
     if (stateKeys.includes(key)) {
       const signalKey = key as keyof State;
-      if (currentState[signalKey] !== newState[signalKey]) {
+      const currentValue = currentState
+        ? currentState[signalKey]
+        : untracked(() => signals[signalKey]());
+      if (currentValue !== newState[signalKey]) {
         signals[signalKey].set(newState[signalKey]);
       }
     } else if (typeof ngDevMode !== 'undefined' && ngDevMode) {