Check if argument passed to isset() is not a variable

[Nitamet]

Added the condition to IssetAnalyzer to emit the issue when anything other than a variable is passed to isset()
Fixes #9949

[weirdan]

@Nitamet thanks for your effort. Please target 5.x (current default) branch.

Apparently, this needs some more work, so I've marked this PR as a draft. Mark it as ready for review when you think you're done with it.

[Nitamet]

And I thought that running composer tests was enough 💀
Yeah, it seems that we should at least take checking array elements into account and maybe there are other cases.
I'll take a closer look at the Expr types and try to fix these problems.

[weirdan]

You need to rebase your changes on top of 5.x and force-push. Just changing the target branch added a bunch of master-only changes we're not going to push to 5.x (they are for Psalm 6).

[Nitamet]

I've added other valid statements and changed the branch, tests seem to be ok now, the failed one is not related to this PR.

There's a little issue related to null coalescing operator though
php.net states that foo ?? bar is just a shorthand for isset(foo) ? foo : bar, but in fact there are differences:

class Foo {
    public const BAR = 'bar';
}

// This works
Foo::BAR ?? "doesn't exist";
// And this crashes
isset(Foo::BAR) ? Foo::BAR : "doesn't exist";

$var = 0;
// This works
($a =& $var) ?? "hello";
// And this crashes
isset((a =& $var)) ? (a =& $var) : "hello";

So if we don't add ClassConstFetch and AssignRef as valid arguments, we will get errors on valid null coalescing operators like in the example above.
And vice versa, if we add them, we won't get errors for invalid arguments ClassConstFetch and AssignRef in isset().
I'm not sure if there's an easy way to distinguish between ?? and isset and if that's in the scope of this PR, so for now I've just added them both as valid to avoid getting false errors.

[weirdan]

so for now I've just added them both as valid to avoid getting false errors.

Sounds good to me.

[weirdan]

Thanks!