[typer] abstract constructors shouldn't be allowed to return a value

[Gama11]

(extracted from #7751)

This compiles, despite the :Void type hint on new:

class Main {
	public static function main() {
		new Test(0);
	}
}

abstract Test(Int) from Int {
	public function new(i:Int):Void {
		this = i;
		return i;
		// return; // Void should be Test
	}
}

[Simn]

I'm removing the 4.0 milestone from this because I don't find this clear cut. Why should class constructors be allowed to return early but abstract constructors not? I think the actual problem here is with the typing.

[Gama11]

I don't think anybody wanted to disallow returns, but it should probably be return;, not return value;? Quoting @ncannasse:

they should not be allowed to return any value

[Simn]

I see, I misread that. Still not 4.0 priority though, and also not easy to fix because this becomes a very normal function to the typer...

[Gama11]

Agreed, doesn't seem urgent.

[Gama11]

Worryingly, a return also seems to bypass the missing this = value check.

class Main {
	public static function main() {
		new Test(0);
	}
}

abstract Test(Int) from Int {
	public function new(i:Int):Void {
		return i; // compiles!
	}
}

I'm not sure what the implicaitons of this are, but that might make it Haxe 4 priority after all?

It at least seems to break with inline, which results in a confusing Cannot inline a not final return.