we should keep this logic out of b.set, since it's very specific to accessors

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -255,16 +255,23 @@ export function client_component(source, analysis, options) {
 
 			const key = binding.prop_alias ?? name;
 
-			properties.push(
-				b.get(key, [b.return(b.call(b.id(name)))]),
-				b.set(
-					key,
-					[b.stmt(b.call(b.id(name), b.id('$$value'))), b.stmt(b.call('$.flushSync'))],
-					analysis.runes && binding.initial
-						? /** @type {import('estree').Expression} */ (binding.initial)
-						: undefined
-				)
-			);
+			const getter = b.get(key, [b.return(b.call(b.id(name)))]);
+
+			const setter = b.set(key, [
+				b.stmt(b.call(b.id(name), b.id('$$value'))),
+				b.stmt(b.call('$.flushSync'))
+			]);
+
+			if (binding.initial) {
+				// turn `set foo($$value)` into `set foo($$value = expression)`
+				setter.value.params[0] = {
+					type: 'AssignmentPattern',
+					left: b.id('$$value'),
+					right: /** @type {import('estree').Expression} */ (binding.initial)
+				};
+			}
+
+			properties.push(getter, setter);
 		}
 	}
 

packages/svelte/src/compiler/utils/builders.js

@@ -210,7 +210,7 @@ export function function_declaration(id, params, body) {
 /**
  * @param {string} name
  * @param {import('estree').Statement[]} body
- * @returns {import('estree').Property}
+ * @returns {import('estree').Property & { value: import('estree').FunctionExpression}}}
  */
 export function get(name, body) {
 	return prop('get', key(name), function_builder(null, [], block(body)));
@@ -305,11 +305,12 @@ export function object_pattern(properties) {
 }
 
 /**
+ * @template {import('estree').Expression} Value
  * @param {'init' | 'get' | 'set'} kind
  * @param {import('estree').Expression} key
- * @param {import('estree').Expression} value
+ * @param {Value} value
  * @param {boolean} computed
- * @returns {import('estree').Property}
+ * @returns {import('estree').Property & { value: Value }}
  */
 export function prop(kind, key, value, computed = false) {
 	return { type: 'Property', kind, key, value, method: false, shorthand: false, computed };
@@ -355,23 +356,10 @@ export function sequence(expressions) {
 /**
  * @param {string} name
  * @param {import('estree').Statement[]} body
- * @param {import('estree').Expression | null} [fallback]
- * @returns {import('estree').Property}
+ * @returns {import('estree').Property & { value: import('estree').FunctionExpression}}
  */
-export function set(name, body, fallback) {
-	return prop(
-		'set',
-		key(name),
-		function_builder(
-			null,
-			[
-				fallback
-					? { type: 'AssignmentPattern', left: id('$$value'), right: fallback }
-					: id('$$value')
-			],
-			block(body)
-		)
-	);
+export function set(name, body) {
+	return prop('set', key(name), function_builder(null, [id('$$value')], block(body)));
 }
 
 /**