fix

packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js

@@ -49,6 +49,15 @@ function build_assignment(operator, left, right, context) {
 					/** @type {Expression} */ (context.visit(right))
 				);
 			}
+		} else if (
+			field &&
+			(field.type === '$derived' || field.type === '$derived.by') &&
+			left.property.type === 'PrivateIdentifier'
+		) {
+			let value = /** @type {Expression} */ (
+				context.visit(build_assignment_value(operator, left, right))
+			);
+			return b.call(b.member(b.this, name), value);
 		}
 	}