[ExpressionLanguage] Null-Coalescing operator supporting non-existent data

[adamkiss]

Potential doc issue for symfony/symfony#54359

Docs suggest that Expression Language supports null-coalescing operator for variables not passed in:

$data = ['exists' => 1];
$language->evaluate('doesnt_exist ?? 2', $data);

But it was suggested to me in the linked issue that the code doesn't seem to actually support that, and that null coalescing operator also isn't mentioned in https://symfony.com/doc/current/reference/formats/expression_language.html#supported-operators

[stof]

To be exact, the if it exists part is true when accessing potentially-missing array keys. But accessing a non-existent variable won't be allowed even with this operator (side note: this is also the way this operator works in JS, where a ?? 'fallback' will still throw a ReferenceError if the variable a does not exist)

[adamkiss]

@stof It specifically calls out "like PHP null-coalescing", where $never_defined ?? 'fallback' is perfectly valid thing.

[stof]

Yeah, this part should be updated. I'm not saying that the doc is correct right now. But the part saying if it exists must not be removed entirely either.