fix: allow mutation of private derived state

Author: paoloricciuti

.changeset/rare-carpets-wave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow mutation of private derived state

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

@@ -35,20 +35,13 @@ export function validate_assignment(node, argument, state) {
 		}
 	}
 
-	let object = /** @type {Expression | Super} */ (argument);
-
-	/** @type {Expression | PrivateIdentifier | null} */
-	let property = null;
-
-	while (object.type === 'MemberExpression') {
-		property = object.property;
-		object = object.object;
-	}
-
-	if (object.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
-		if (state.private_derived_state.includes(property.name)) {
-			e.constant_assignment(node, 'derived state');
-		}
+	if (
+		argument.type === 'MemberExpression' &&
+		argument.object.type === 'ThisExpression' &&
+		argument.property.type === 'PrivateIdentifier' &&
+		state.private_derived_state.includes(argument.property.name)
+	) {
+		e.constant_assignment(node, 'derived state');
 	}
 }
 

packages/svelte/tests/validator/samples/mutate-derived-private-field/errors.json

@@ -0,0 +1 @@
+[]

packages/svelte/tests/validator/samples/mutate-derived-private-field/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		#der = $derived({test: 0});
+
+		set test(v){
+			this.#der.test = 45;
+		}
+	}
+</script>

packages/svelte/tests/validator/samples/reassign-derived-private-field/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "constant_assignment",
+		"message": "Cannot assign to derived state",
+		"start": {
+			"column": 3,
+			"line": 6
+		},
+		"end": {
+			"column": 27,
+			"line": 6
+		}
+	}
+]

packages/svelte/tests/validator/samples/reassign-derived-private-field/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		#der = $derived({ test: 0 });
+
+		set test(v){
+			this.#der = { test: 45 };
+		}
+	}
+</script>