refactor(core): expose SIGNAL_NODE to allow for advanced extensions…

… of signals (angular#53571)

For the implementation of input signals, we want to extend the signal
primitive. The basic methods exposed here are not suitable as we'd like
to store additional metadata on the reactive node, and also have a
custom getter (for required inputs and throwing).

To enable this, one small piece was missing. This commit exposes it and
also improves type safety, now that `SignalNode` is typed properly after
the previous commit.

PR Close angular#53571

goldens/public-api/core/primitives/signals/index.md

@@ -95,6 +95,9 @@ export function setThrowInvalidWriteToSignalError(fn: () => never): void;
 // @public
 export const SIGNAL: unique symbol;
 
+// @public (undocumented)
+export const SIGNAL_NODE: SignalNode<unknown>;
+
 // @public (undocumented)
 export interface SignalGetter<T> extends SignalBaseGetter<T> {
     // (undocumented)

packages/core/primitives/signals/index.ts

@@ -10,6 +10,6 @@ export {createComputed} from './src/computed';
 export {defaultEquals, ValueEqualityFn} from './src/equality';
 export {setThrowInvalidWriteToSignalError} from './src/errors';
 export {consumerAfterComputation, consumerBeforeComputation, consumerDestroy, consumerPollProducersForChange, getActiveConsumer, isInNotificationPhase, isReactive, producerAccessed, producerNotifyConsumers, producerUpdatesAllowed, producerUpdateValueVersion, Reactive, REACTIVE_NODE, ReactiveNode, setActiveConsumer, SIGNAL} from './src/graph';
-export {createSignal, setPostSignalSetFn, SignalGetter, signalMutateFn, SignalNode, signalSetFn, signalUpdateFn} from './src/signal';
+export {createSignal, setPostSignalSetFn, SIGNAL_NODE, SignalGetter, signalMutateFn, SignalNode, signalSetFn, signalUpdateFn} from './src/signal';
 export {createWatch, Watch, WatchCleanupFn, WatchCleanupRegisterFn} from './src/watch';
 export {setAlternateWeakRefImpl} from './src/weak_ref';

packages/core/primitives/signals/src/signal.ts

@@ -91,7 +91,7 @@ export function signalMutateFn<T>(node: SignalNode<T>, mutator: (value: T) => vo
 // Note: Using an IIFE here to ensure that the spread assignment is not considered
 // a side-effect, ending up preserving `COMPUTED_NODE` and `REACTIVE_NODE`.
 // TODO: remove when https://github.com/evanw/esbuild/issues/3392 is resolved.
-const SIGNAL_NODE: object = /* @__PURE__ */ (() => {
+export const SIGNAL_NODE: SignalNode<unknown> = /* @__PURE__ */ (() => {
   return {
     ...REACTIVE_NODE,
     equal: defaultEquals,