[coro?] Some type inference weirdness

```haxe
import haxe.coro.Coroutine;

typedef Yield = Coroutine R>;

function create(f:Coroutine -> Void>) {}

function main() {
create(f -> {
var sum = 1;
sum += f(sum); // Float should be Int
});
}
```

I don't know where that `Float` comes from, seems like the compiler defaults to it and then complains about itself.