Bug in async-await transformer when using async generators

[jridgewell]

I'm using esbuild to transform async/await into a promise+generator to support async zone support (we use a patched Promise.prototype.then to snapshot/restore the async zone). I'm passing --supported:async-await=false, which works correctly for regular async functions. But it transforms async generators into an incorrect result.

The __async helper is consuming the entire generator without yielding to the caller when it encounters a yield in the wrapped async generator. I later realized that setting --support:async-generator=false will actually throw an error at build time because downleveling is unsupported. I think transforming async generators when async-await=false is a bug, but ideally I'd like to see support for downleveling async generators.

// input.js
async function* foo() {
  await Promise.resolve(1);

  const [a, b] = yield 2;
  return a + b;
}

async function run() {
  const g = foo();

  let next = await g.next();
  console.assert(next.value === 2);
  console.assert(next.done === false);

  next = await g.next([1, 2]);
  console.assert(next.value === 3);
  console.assert(next.done === true);
}

run();

// output.js
var __async = (__this, __arguments, generator) => {
  return new Promise((resolve, reject) => {
    var fulfilled = (value) => {
      try {
        step(generator.next(value));
      } catch (e) {
        reject(e);
      }
    };
    var rejected = (value) => {
      try {
        step(generator.throw(value));
      } catch (e) {
        reject(e);
      }
    };
    var step = (x) => x.done ? resolve(x.value) : Promise.resolve(x.value).then(fulfilled, rejected);
    step((generator = generator.apply(__this, __arguments)).next());
  });
};
function* foo() {
  return __async(this, null, function* () {
    yield Promise.resolve(1);
    const [a, b] = yield 2;
    return a + b;
  });
}
function run() {
  return __async(this, null, function* () {
    const g = foo();
    let next = yield g.next();
    console.assert(next.value === 2);
    console.assert(next.done === false);
    next = yield g.next([1, 2]);
    console.assert(next.value === 3);
    console.assert(next.done === true);
  });
}
run();

[evanw]

This bug is due to you telling esbuild that async generator functions are supported natively but that async isn't supported natively, which is nonsensical. I can fix the bug to make this a build error so that esbuild no longer lets you do this.