fix: treat concise arrow function bodies as implicit returns when calculating blockers (#18613)

Fixes #18612.

A concise arrow body does not contain a `ReturnStatement` which we traverse in full calculate_blockers (else we bail on functions), so short-cut to `touch` there.


---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/light-pandas-attack.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: treat concise arrow function bodies as implicit returns when calculating blockers

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1235,12 +1235,15 @@ function calculate_blockers(instance, analysis) {
 				? /** @type {ESTree.FunctionExpression | ESTree.ArrowFunctionExpression} */ (fn.init)
 				: fn;
 
-		trace_references(
-			init.body,
-			reads_writes,
-			reads_writes,
-			/** @type {Scope} */ (instance.scopes.get(init))
-		);
+		const fn_scope = /** @type {Scope} */ (instance.scopes.get(init));
+
+		if (init.body.type === 'BlockStatement') {
+			trace_references(init.body, reads_writes, reads_writes, fn_scope);
+		} else {
+			// A concise arrow body is an implicit return, so treat it like the
+			// `ReturnStatement` visitor in `trace_references` would.
+			touch(init.body, fn_scope, reads_writes);
+		}
 
 		const max = [...reads_writes].reduce((max, binding) => {
 			if (binding.blocker) {

packages/svelte/tests/runtime-runes/samples/async-bind-factory-function-remote/_config.js

@@ -3,12 +3,12 @@ import { test } from '../../test';
 
 export default test({
 	mode: ['async-server', 'client', 'hydrate'],
-	ssrHtml: 'true true true true true',
+	ssrHtml: 'true true true true true true',
 
 	async test({ assert, target }) {
 		await new Promise((resolve) => setTimeout(resolve, 10));
 		await tick();
 
-		assert.htmlEqual(target.innerHTML, 'true true true true true');
+		assert.htmlEqual(target.innerHTML, 'true true true true true true');
 	}
 });

packages/svelte/tests/runtime-runes/samples/async-bind-factory-function-remote/main.svelte

@@ -21,6 +21,8 @@
 		const indirect = () => checkedFactory()();
 		return indirect;
 	}
+
+	const arrow = () => () => checked;
 </script>
 
 <!-- force into separate effects -->
@@ -39,3 +41,6 @@
 {#if true}
 	{indirectChecked2()()}
 {/if}
+{#if true}
+	{arrow()()}
+{/if}