Why can't you call it on a function?

I think that the only possible solution for invokeArgs working for closures is to build PHP code from AST and use eval. And it will fail anyway for code like this:

<?php

class Foo
{
    private $foo = 'foo';
    
    public function boo()
    {
        return (function () {
            return $this->foo;
        })();
    }
}

Ah, got it. Didn't consider that the closure wasn't loaded yet.