fix: take private and public into account for `constant_assignment` of derived state (#15276)

Fixes #15273

.changeset/five-apes-develop.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: take private and public into account for `constant_assignment` of derived state

packages/svelte/src/compiler/phases/2-analyze/types.d.ts

@@ -19,7 +19,7 @@ export interface AnalysisState {
 	component_slots: Set<string>;
 	/** Information about the current expression/directive/block value */
 	expression: ExpressionMetadata | null;
-	derived_state: string[];
+	derived_state: { name: string; private: boolean }[];
 	function_depth: number;
 
 	// legacy stuff

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

@@ -7,7 +7,7 @@ import { get_rune } from '../../scope.js';
  * @param {Context} context
  */
 export function ClassBody(node, context) {
-	/** @type {string[]} */
+	/** @type {{name: string, private: boolean}[]} */
 	const derived_state = [];
 
 	for (const definition of node.body) {
@@ -18,7 +18,10 @@ export function ClassBody(node, context) {
 		) {
 			const rune = get_rune(definition.value, context.state.scope);
 			if (rune === '$derived' || rune === '$derived.by') {
-				derived_state.push(definition.key.name);
+				derived_state.push({
+					name: definition.key.name,
+					private: definition.key.type === 'PrivateIdentifier'
+				});
 			}
 		}
 	}

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

@@ -1,4 +1,4 @@
-/** @import { AssignmentExpression, Expression, Literal, Node, Pattern, PrivateIdentifier, Super, UpdateExpression, VariableDeclarator } from 'estree' */
+/** @import { AssignmentExpression, Expression, Identifier, Literal, Node, Pattern, PrivateIdentifier, Super, UpdateExpression, VariableDeclarator } from 'estree' */
 /** @import { AST, Binding } from '#compiler' */
 /** @import { AnalysisState, Context } from '../../types' */
 /** @import { Scope } from '../../../scope' */
@@ -38,16 +38,22 @@ export function validate_assignment(node, argument, state) {
 			e.snippet_parameter_assignment(node);
 		}
 	}
-
 	if (
 		argument.type === 'MemberExpression' &&
 		argument.object.type === 'ThisExpression' &&
 		(((argument.property.type === 'PrivateIdentifier' || argument.property.type === 'Identifier') &&
-			state.derived_state.includes(argument.property.name)) ||
+			state.derived_state.some(
+				(derived) =>
+					derived.name === /** @type {PrivateIdentifier | Identifier} */ (argument.property).name &&
+					derived.private === (argument.property.type === 'PrivateIdentifier')
+			)) ||
 			(argument.property.type === 'Literal' &&
 				argument.property.value &&
 				typeof argument.property.value === 'string' &&
-				state.derived_state.includes(argument.property.value)))
+				state.derived_state.some(
+					(derived) =>
+						derived.name === /** @type {Literal} */ (argument.property).value && !derived.private
+				)))
 	) {
 		e.constant_assignment(node, 'derived state');
 	}

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

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

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

@@ -0,0 +1,13 @@
+<script>
+	class Test {
+	  #deps = () => [];
+	
+	  deps = $derived.by(() => {
+		return [];
+	  });		
+			
+	  constructor(f = () => []) {
+		this.#deps = f;
+	  }
+	}
+  </script>