test(transformer/async-to-generator): failing test for async arrow function in class static block

[overlookmotel]

It's arguable whether we should support this, because async functions is ES2018 and class static blocks is ES2022. So there should not be any browser which needs async-to-generator transform, and not the static block transform too. And when class-static-block transform is enabled, this works fine.

But personally, I think we should support it, because:

1. We allow user to enable/disable arbitrary transforms via TransformOptions.
2. We support this in async arrow functions in class static blocks, so it makes sense to complete that support.
3. I don't think it should be too hard, as the framework for it is already there in the async-to-generator transform.

Babel REPL

[overlookmotel]

• test(transformer/async-to-generator): failing test for async arrow function in class static block #8387 👈 (View in Graphite)
• main

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[Dunqing]

Just found that Babel's output is incorrect, super can't be outside a class. It wasted a few hours for me.

[Dunqing]

Even if the async arrow function is not in the class static block, it also needs to transform super, because async-to-generator will transform the async arrow function to a function generator, thus super will be in the wrong place.

But there is a troublesome example

Input:

class C { static f = async () => super.prop;  }

Babel's Output:

var _superprop_getProp = () => super.prop;
class C {
  static f = babelHelpers.asyncToGenerator(function* () {
    return _superprop_getProp();
  });
}

Babel's implementation is wrong because it inserts _superprop_getProp outside the class. But how to fix it? Where is the correct place to insert _superprop_getProp?

[overlookmotel]

Just found that Babel's output is incorrect, super can't be outside a class. It wasted a few hours for me.

Oh bollocks sorry my fault. I posted the Babel REPL link meaning to say "look Babel is wrong again!" but I forgot to actually write that. The test in this PR shows what I think the correct output is for static blocks.

For static blocks, I don't expect it's too hard because there's already a block for the var statement to go in, and the transform already correctly handles this correctly in static blocks: class C { static { async () => this; } }

[overlookmotel]

For properties (instance prop or static prop), it's much more annoying. Babel's solution for instance props looks correct, but for some reason it gets it wrong for static props: Babel REPL

But I'm not sure we should attempt to cover async arrow functions in class properties. It's maybe not worth the work. I only suggested that we might want to deal with static blocks because I thought it'd be quite easy.

We also have this problem with other transforms. e.g. class C { prop = x ?? y; } should probably create the temp var inside the property initializer, not outside the class. Or maybe in this cases it's fine (Babel thinks so), but in other transforms it's probably not fine.

We have other weird cases where we need to create temp vars as part of a transform inside:

• function f(x = somethingThatNeedsALocalTempVarWhenTransformed) {}
• for (let [x = somethingThatNeedsALocalTempVarWhenTransformed] of arr) {}
• Probably more

I put class property initializers in the same category.

Rather than tackling this in individual transforms, I think we should probably try to figure out a way to handle these problems universally with helpers that create temp vars, and handle these edge cases for us.

[overlookmotel]

It may also be easier to deal with when we have our own helper library. e.g. something like:

class C {
  static f = async (x, y) => super.prop;
}

->

class C {
  static f = oxcHelpers.asyncToGenerator(
    function*(x, y, _superprop_getProp) {
      return _superprop_getProp();
    },
    () => super.prop
  );
}

[Dunqing]

It may also be easier to deal with when we have our own helper library. e.g. something like:

class C {
  static f = async (x, y) => super.prop;
}

->

class C {
  static f = oxcHelpers.asyncToGenerator(
    function*(x, y, _superprop_getProp) {
      return _superprop_getProp();
    },
    () => super.prop
  );
}

I just came up with a way by SequenceExpression

var _superprop_getProp;
class C {
  static f =
    ((_superprop_getProp = () => super.prop),
    /*#__PURE__*/ (function () {
      var _ref = babelHelpers.asyncToGenerator(function* (x, y) {
        return _superprop_getProp();
      });
      return function (_x, _x2) {
        return _ref.apply(this, arguments);
      };
    })());
}

It doesn't rely on our own helper, so we can support it soon.