Adjust ReadOnlySignal so we cant widen the type

.changeset/seven-seahorses-cry.md

@@ -0,0 +1,16 @@
+---
+"@preact/signals-core": minor
+---
+
+Adjust the `ReadOnlySignal` type to not inherit from `Signal`
+this way the type can't be widened without noticing, i.e. when
+we'd have
+
+```js
+const sig: Signal = useComputed(() => x);
+```
+
+We would have widened the type to be mutable again, which for
+a computed is not allowed. We want to provide the tools to our
+users to avoid these footguns hence we are correcting this type
+in a minor version.

packages/core/src/index.ts

@@ -657,8 +657,15 @@ Object.defineProperty(Computed.prototype, "value", {
 /**
  * An interface for read-only signals.
  */
-interface ReadonlySignal<T = any> extends Signal<T> {
+interface ReadonlySignal<T = any> {
 	readonly value: T;
+	peek(): T;
+
+	subscribe(fn: (value: T) => void): () => void;
+	valueOf(): T;
+	toString(): string;
+	toJSON(): T;
+	brand: typeof BRAND_SYMBOL;
 }
 
 /**

packages/core/test/signal.test.tsx

@@ -5,6 +5,7 @@ import {
 	batch,
 	Signal,
 	untracked,
+	ReadonlySignal,
 } from "@preact/signals-core";
 
 describe("signal", () => {
@@ -962,15 +963,15 @@ describe("computed()", () => {
 	});
 
 	it("should detect simple dependency cycles", () => {
-		const a: Signal = computed(() => a.value);
+		const a: ReadonlySignal = computed(() => a.value);
 		expect(() => a.value).to.throw(/Cycle detected/);
 	});
 
 	it("should detect deep dependency cycles", () => {
-		const a: Signal = computed(() => b.value);
-		const b: Signal = computed(() => c.value);
-		const c: Signal = computed(() => d.value);
-		const d: Signal = computed(() => a.value);
+		const a: ReadonlySignal = computed(() => b.value);
+		const b: ReadonlySignal = computed(() => c.value);
+		const c: ReadonlySignal = computed(() => d.value);
+		const d: ReadonlySignal = computed(() => a.value);
 		expect(() => a.value).to.throw(/Cycle detected/);
 	});
 
@@ -1243,15 +1244,15 @@ describe("computed()", () => {
 		});
 
 		it("should detect simple dependency cycles", () => {
-			const a: Signal = computed(() => a.peek());
+			const a: ReadonlySignal = computed(() => a.peek());
 			expect(() => a.peek()).to.throw(/Cycle detected/);
 		});
 
 		it("should detect deep dependency cycles", () => {
-			const a: Signal = computed(() => b.value);
-			const b: Signal = computed(() => c.value);
-			const c: Signal = computed(() => d.value);
-			const d: Signal = computed(() => a.peek());
+			const a: ReadonlySignal = computed(() => b.value);
+			const b: ReadonlySignal = computed(() => c.value);
+			const c: ReadonlySignal = computed(() => d.value);
+			const d: ReadonlySignal = computed(() => a.peek());
 			expect(() => a.peek()).to.throw(/Cycle detected/);
 		});
 
@@ -1340,7 +1341,7 @@ describe("computed()", () => {
 		it("should be garbage collectable after it has lost all of its listeners", async () => {
 			const s = signal(0);
 
-			let ref: WeakRef<Signal>;
+			let ref: WeakRef<ReadonlySignal>;
 			let dispose: () => void;
 			(function () {
 				const c = computed(() => s.value);

packages/preact/test/index.test.tsx

@@ -382,7 +382,7 @@ describe("@preact/signals", () => {
 			const childSpy = sinon.spy();
 			const parentSpy = sinon.spy();
 
-			function Child({ num }: { num: Signal<number> }) {
+			function Child({ num }: { num: ReadonlySignal<number> }) {
 				childSpy();
 				return <p>{num.value}</p>;
 			}

packages/react/src/index.ts

@@ -39,4 +39,7 @@ declare module "@preact/signals-core" {
 	// @ts-ignore internal Signal is viewed as function
 	// eslint-disable-next-line @typescript-eslint/no-empty-interface
 	interface Signal extends ReactElement {}
+	// @ts-ignore internal Signal is viewed as function
+	// eslint-disable-next-line @typescript-eslint/no-empty-interface
+	interface ReadonlySignal extends ReactElement {}
 }

packages/react/test/shared/updates.tsx

@@ -630,7 +630,7 @@ export function updateSignalsTests(usingTransform = false) {
 			const childSpy = sinon.spy();
 			const parentSpy = sinon.spy();
 
-			function Child({ num }: { num: Signal<number> }) {
+			function Child({ num }: { num: ReadonlySignal<number> }) {
 				childSpy();
 				return <p>{num.value}</p>;
 			}