chore: add warning for derived self referencin

.changeset/new-cooks-roll.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: add warning for derived self referencing

packages/svelte/messages/compile-warnings/script.md

@@ -2,6 +2,10 @@
 
 > Use `$derived.by(() => {...})` instead of `$derived((() => {...})())`
 
+## derived_referenced_self
+
+> A derived value should not reference itself.
+
 ## export_let_unused
 
 > Component has unused export property '%name%'. If it is for external reference only, please consider using `export const %name%`

packages/svelte/src/compiler/phases/2-analyze/visitors/Identifier.js

@@ -65,7 +65,16 @@ export function Identifier(node, context) {
 
 	let binding = context.state.scope.get(node.name);
 
-	if (!context.state.analysis.runes) {
+	if (context.state.analysis.runes) {
+		// Check for recursive references of a derived from within a derived and issue a warning.
+		if (
+			binding?.kind === 'derived' &&
+			node !== binding.node &&
+			context.path.find((n) => n.type === 'VariableDeclarator' && n.init === binding.initial)
+		) {
+			w.derived_referenced_self(node);
+		}
+	} else {
 		if (node.name === '$$props') {
 			context.state.analysis.uses_props = true;
 		}

packages/svelte/src/compiler/warnings.js

@@ -103,6 +103,7 @@ export const codes = [
 	"reactive_declaration_invalid_placement",
 	"reactive_declaration_module_script",
 	"state_referenced_locally",
+	"derived_referenced_self",
 	"store_rune_conflict",
 	"css_unused_selector",
 	"attribute_avoid_is",
@@ -644,6 +645,14 @@ export function state_referenced_locally(node) {
 	w(node, "state_referenced_locally", "State referenced in its own scope will never update. Did you mean to reference it inside a closure?");
 }
 
+/**
+ * A derived value should not reference itself.
+ * @param {null | NodeLike} node
+ */
+export function derived_referenced_self(node) {
+	w(node, "derived_referenced_self", "A derived value should not reference itself.");
+}
+
 /**
  * It looks like you're using the `$%name%` rune, but there is a local binding called `%name%`. Referencing a local variable with a `$` prefix will create a store subscription. Please rename `%name%` to avoid the ambiguity
  * @param {null | NodeLike} node

packages/svelte/tests/validator/samples/derived-reference-self/input.svelte

@@ -0,0 +1,26 @@
+<script>
+	let count = $state(0);
+	let disabled = $state(false);
+
+	let even = $derived.by(() => {
+		if (disabled) return even;
+
+		return count % 2 === 0;
+	})
+
+	function increment() {
+		count += 1;
+	}
+</script>
+
+<p>Is {even ? 'even' : 'odd'}</p>
+
+<button onclick={increment}>
+	clicks: {count}
+</button>
+
+<label>
+	<input type="checkbox" bind:checked={disabled} />
+
+	Disable
+</label>

packages/svelte/tests/validator/samples/derived-reference-self/warnings.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "derived_referenced_self",
+		"message": "A derived value should not reference itself.",
+		"start": {
+			"column": 23,
+			"line": 6
+		},
+		"end": {
+			"column": 27,
+			"line": 6
+		}
+	}
+]