[js][es6] Unexpected "Must call super() constructor before accessing this"

[haxiomic]

Hey @nadako, just want to say absolutely loving the new es6 class output! It's been working well for me so far but I've run into an issue with a class like this:

class Subclass extends js.html.Uint8Array {

	var member: Float;

	function new() {
		super(1);

		js.Browser.window.addEventListener('mousemove', () -> {
			member = 5;
		});
	}

}

The error is

src/Main.hx:13: lines 13-19 : Must call `super()` constructor before accessing `this` in classes derived from an extern class with constructor

It seems to happen when a member is accessed within a closure

For example, it also happens with

	var member: Float;
	function new() {
		super(1);

		function x() {
			member = 2;
		}
         }

(Haxe build 4eefb97)

[nadako]

Err, var _gthis = this gets in the way here. @Simn why is that generated at the very top of the function instead of before the first place where it's needed?

[Simn]

Because that's how it's implemented at the moment.

[Simn]

I'm a bit nervous about changing this in the typer. While we can easily detect the place where it's needed at the time of typing, I'm not sure if there could be transformations which require a this variable earlier. It then makes sense to handle this during post-processing, at a point where we can be sure that nothing inserts itself before our this variable.

We might even want to handle this in the generator itself to avoid an extra filter.

[Simn]

Then again, given that this is a normal variable, I don't think anything could be inserted there anyway.

[Simn]

That should do it. Assigning to nadako to clean up after me and add a test.

[sonygod]

https://community.haxe.org/t/js-es6-must-call-super-constructor-before-accessing-this-in-classes-derived-from-an-extern-class-with-constructor/2870