fix(core): allow toSignal calls in reactive context

packages/core/rxjs-interop/src/to_signal.ts

@@ -175,21 +175,23 @@ export function toSignal<T, U = undefined>(
     state = signal<State<T|U>>({kind: StateKind.Value, value: options?.initialValue as U});
   }
 
-  const sub = source.subscribe({
-    next: value => state.set({kind: StateKind.Value, value}),
-    error: error => state.set({kind: StateKind.Error, error}),
-    // Completion of the Observable is meaningless to the signal. Signals don't have a concept of
-    // "complete".
-  });
-
-  if (ngDevMode && options?.requireSync && untracked(state).kind === StateKind.NoValue) {
-    throw new ɵRuntimeError(
-        ɵRuntimeErrorCode.REQUIRE_SYNC_WITHOUT_SYNC_EMIT,
-        '`toSignal()` called with `requireSync` but `Observable` did not emit synchronously.');
-  }
+  untracked(() => {
+    const sub = source.subscribe({
+      next: value => state.set({kind: StateKind.Value, value}),
+      error: error => state.set({kind: StateKind.Error, error}),
+      // Completion of the Observable is meaningless to the signal. Signals don't have a concept of
+      // "complete".
+    });
+
+    if (ngDevMode && options?.requireSync && state().kind === StateKind.NoValue) {
+      throw new ɵRuntimeError(
+          ɵRuntimeErrorCode.REQUIRE_SYNC_WITHOUT_SYNC_EMIT,
+          '`toSignal()` called with `requireSync` but `Observable` did not emit synchronously.');
+    }
 
-  // Unsubscribe when the current context is destroyed, if requested.
-  cleanupRef?.onDestroy(sub.unsubscribe.bind(sub));
+    // Unsubscribe when the current context is destroyed, if requested.
+    cleanupRef?.onDestroy(sub.unsubscribe.bind(sub));
+  });
 
   // The actual returned signal is a `computed` of the `State` signal, which maps the various states
   // to either values or errors.

packages/core/rxjs-interop/test/to_signal_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {DestroyRef, EnvironmentInjector, Injector, runInInjectionContext} from '@angular/core';
+import {computed, EnvironmentInjector, Injector, runInInjectionContext} from '@angular/core';
 import {toSignal} from '@angular/core/rxjs-interop';
 import {BehaviorSubject, ReplaySubject, Subject} from 'rxjs';
 
@@ -109,6 +109,19 @@ describe('toSignal()', () => {
     expect(counter()).toBe(1);
   });
 
+  it('should allow toSignal creation in a reactive context - issue 51027', () => {
+    const counter$ = new BehaviorSubject(1);
+
+    const injector = Injector.create([]);
+
+    const doubleCounter = computed(() => {
+      const counter = toSignal(counter$, {requireSync: true, injector});
+      return counter() * 2;
+    });
+
+    expect(doubleCounter()).toBe(2);
+  });
+
   describe('with no initial value', () => {
     it('should return `undefined` if read before a value is emitted', test(() => {
          const counter$ = new Subject<number>();

packages/core/test/signals/computed_spec.ts

@@ -167,7 +167,16 @@ describe('computed', () => {
     expect(watchCount).toEqual(2);
   });
 
-  it('should disallow writing to signals within computeds', () => {
+  it('should allow signal creation within computed', () => {
+    const doubleCounter = computed(() => {
+      const counter = signal(1);
+      return counter() * 2;
+    });
+
+    expect(doubleCounter()).toBe(2);
+  });
+
+  it('should disallow writing to signals within computed', () => {
     const source = signal(0);
     const illegal = computed(() => {
       source.set(1);