[VOTE] Make compact function reports undefined passed variables

The existence of anything that internally relies on get_defined_vars() is a
blocker for applying further optimisations to the engine (think stack frames),
which is probably why Dmitry suggested its removal.

I did not know it was an optimization issue.

Accessing variables by name is a problem for optimimisation and JIT-ing, because we lose data-flow dependencies. But, this is not the end of the world. We just don't optimize functions that use this "bad" pattern.

Thanks. Dmitry.

I suppose I could substitute a userland equivalent:

function compact_(array $compact, array $vars)
{
return array_intersect_key($vars, array_flip($compact));
}

which changes A::c() to

function c()
{
$d = $this->d();
$a = pow($d, $d + 1);
$c = $a ^ 0b1100;
$b = $a - $d;

return new B(
    compact_(
        $this->b(),
        `get_defined_vars()`
    )
);

}

That is not so bad. As far as I am concerned, deprecate away.