From b34a8a38c649275e47845fd7415477c539c47506 Mon Sep 17 00:00:00 2001
From: Shannon Booth <shannon@serenityos.org>
Date: Sun, 6 Apr 2025 23:36:40 +1200
Subject: [PATCH] LibJS: Do not generate implicit await for async functions

Despite the comment in our source, as far as I can tell from my
testing, other engines do not appear to do this.

For example, the result of the following test now matches node and
LibJS by logging `0` calls.

```
async function myFunction() {
    let calls = 0;
    function makeConstructorObservable(promise) {
        Object.defineProperty(promise, "constructor", {
            get() {
                calls++;
                return Promise;
            },
        });
        return promise;
    }

    async function test() {
        try {
            return makeConstructorObservable(Promise.reject(1));
        } catch {
            return 2;
        }
    }
    test().catch(() => { });

    console.log(calls);
};

myFunction()
```

We regress in one test262 Array.fromAsync test, but as far as I can
tell this is due to some unrelated issue in the engine.

Fixes #3251
---
 Libraries/LibJS/Bytecode/ASTCodegen.cpp     |  7 +------
 Libraries/LibJS/Tests/syntax/async-await.js | 20 ++++++++++----------
 2 files changed, 11 insertions(+), 16 deletions(-)

diff --git a/Libraries/LibJS/Bytecode/ASTCodegen.cpp b/Libraries/LibJS/Bytecode/ASTCodegen.cpp
index 85d9f61f2e2..ae15e368a68 100644
--- a/Libraries/LibJS/Bytecode/ASTCodegen.cpp
+++ b/Libraries/LibJS/Bytecode/ASTCodegen.cpp
@@ -1852,12 +1852,7 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> ReturnStatement::genera
         return_value = TRY(m_argument->generate_bytecode(generator)).value();
 
         //     3. If GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
-        // Spec Issue?: The spec doesn't seem to do implicit await on explicit return for async functions, but does for
-        //              async generators. However, the major engines do so, and this is observable via constructor lookups
-        //              on Promise objects and custom thenables.
-        //              See: https://tc39.es/ecma262/#sec-asyncblockstart
-        //              c. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
-        if (generator.is_in_async_function()) {
+        if (generator.is_in_async_generator_function()) {
             auto received_completion = generator.allocate_register();
             auto received_completion_type = generator.allocate_register();
             auto received_completion_value = generator.allocate_register();
diff --git a/Libraries/LibJS/Tests/syntax/async-await.js b/Libraries/LibJS/Tests/syntax/async-await.js
index 1d5c4bcb486..7ed67de988a 100644
--- a/Libraries/LibJS/Tests/syntax/async-await.js
+++ b/Libraries/LibJS/Tests/syntax/async-await.js
@@ -202,7 +202,7 @@ describe("await cannot be used in class static init blocks", () => {
 });
 
 describe("await thenables", () => {
-    test("async returning a thanable variable without fulfilling", () => {
+    test("async returning a thenable variable without fulfilling", () => {
         let isCalled = false;
         const obj = {
             then() {
@@ -216,7 +216,7 @@ describe("await thenables", () => {
         expect(isCalled).toBe(true);
     });
 
-    test("async returning a thanable variable that fulfills", () => {
+    test("async returning a thenable variable that fulfills", () => {
         let isCalled = false;
         const obj = {
             then(fulfill) {
@@ -240,7 +240,7 @@ describe("await thenables", () => {
         });
         f();
         runQueuedPromiseJobs();
-        expect(isCalled).toBe(true);
+        expect(isCalled).toBe(false);
     });
 
     test("async returning a thenable directly that fulfills", () => {
@@ -253,7 +253,7 @@ describe("await thenables", () => {
         });
         f();
         runQueuedPromiseJobs();
-        expect(isCalled).toBe(true);
+        expect(isCalled).toBe(false);
     });
 });
 
@@ -279,15 +279,10 @@ describe("await observably looks up constructor of Promise objects increasing ca
         try {
             await makeConstructorObservable(Promise.reject(3));
         } catch {}
-        try {
-            return makeConstructorObservable(Promise.reject(1));
-        } catch {
-            return 2;
-        }
     }
     test();
     runQueuedPromiseJobs();
-    expect(calls).toBe(4);
+    expect(calls).toBe(3);
 });
 
 describe("await observably looks up constructor of Promise objects not increasing call count", () => {
@@ -306,6 +301,11 @@ describe("await observably looks up constructor of Promise objects not increasin
         await makeConstructorObservable(new Boolean(true));
         await makeConstructorObservable({});
         await makeConstructorObservable(new Number(2));
+        try {
+            return makeConstructorObservable(Promise.reject(1));
+        } catch {
+            return 2;
+        }
     }
     test();
     runQueuedPromiseJobs();