transformer: Invalid output for stage 3 decorator

[overlookmotel]

Reduced example, boiled down from failures in monitor-oxc: https://github.com/oxc-project/monitor-oxc/actions/runs/12864766675/job/35864033007

Input:

let x;

class C {
  accessor #prop = 123;

  #method() {
    this.#prop;
  }
}

Output:

import _classPrivateMethodInitSpec from "@babel/runtime/helpers/classPrivateMethodInitSpec";
import _classPrivateFieldGet from "@babel/runtime/helpers/classPrivateFieldGet2";

let x;

var _C_brand = new WeakSet();
class C {
    constructor() {
        _classPrivateMethodInitSpec(this, _C_brand);
    }
    accessor #prop = 123;
}

function _method() {
    // Next line is invalid
    _classPrivateFieldGet(, this);
}

Oxc playground

What is really weird is that if you comment out the let x line, then you get different output for _method():

function _method() {
    _classPrivateFieldGet(_C, this);
}

Oxc playground

But that's not correct either! If #prop was a normal property not an accessor, it should be _classPrivateFieldGet(_prop, this);.

The bogus output _classPrivateFieldGet(, this) is in once sense "correct" because we don't support accessor properties, so there is no _prop variable.

But why does whether there's a line let x above the class make any difference to the output? Any other statement which generates a binding also produces the empty output e.g. const x = 1;, var x;, function x() {}, class X {}, but other statements e.g. x = 1 don't.

I suspect there may be 2 different bugs at play here.

[overlookmotel]

Oh I think I figured it out.

We create a dummy BoundIdentifier for the accessor with SymbolId = 0:

oxc/crates/oxc_transformer/src/es2022/class_properties/class.rs

Lines 153 to 163 in 2a2ad53

	ClassElement::AccessorProperty(prop) => {
	// TODO: Not sure what we should do here.
	// Only added this to prevent panics in TS conformance tests.
	if let PropertyKey::PrivateIdentifier(ident) = &prop.key {
	let dummy_binding = BoundIdentifier::new(Atom::empty(), SymbolId::new(0));
	private_props.insert(
	ident.name,
	PrivateProp::new(dummy_binding, prop.r#static, None, true),
	);
	}
	}

The private method transform visits body of the method and replaces all references to the class name with _C. When the class is the first binding in the file, its SymbolId is 0. So it sees the reference to the dummy symbol, says "this is a reference to the class" and replaces it with _C. But if there's let x statement before the class, then the class's SymbolId is 1, so the reference doesn't match the class's SymbolId, and it doesn't replace it. Mystery solved!

So question is: What's the fix?

We could solve the monitor-oxc error easily by making the output valid syntatically (but nonsensical):

- let dummy_binding = BoundIdentifier::new(Atom::empty(), SymbolId::new(0));
+ let dummy_binding = BoundIdentifier::new(Atom::from("DUMMY"), SymbolId::new(0));

But that's not a real fix! Should we do that for now and get monitor-oxc passing, and decorators transform will deal with it properly? Or do legacy decorators not include accessors?

Other options:

1. Emit an "accessors are not supported" error.
2. Treat accessor #prop the same as plain #prop.

Any opinion @Dunqing?

[Boshen]

Thank you for the investigation.

https://github.com/puppeteer/puppeteer/blob/89f4f3b1079874b2dd91c727bc058c1b6e09096e/packages/puppeteer-core/src/bidi/Browser.ts#L103-L104

  @bubble()
  accessor #trustedEmitter = new EventEmitter<BrowserEvents>();

This is stage 3 decorator. accessor = stage 3.

We should throw an "stage 3 decorator not supported" error right now.

[Boshen]

I disabled it in oxc-project/monitor-oxc@28c3d1a

Let's come back to this after stage 3 decorator is implemented.

• add a hard error when stage 3 decorator is encountered. @Dunqing