[php7] null coalescing operator not supported

[fredemmott]

http://3v4l.org/qLYI8

<?php

var_dump(1 ?? 2 ?? 3);
var_dump(1 ?: 2 ?: 3);

[reeze]

Which version of HHVM should have this feature? HHVM3.6? or HHVM3.9? I am interested in implement it but will it get merged if the current master version is not aimed to be compatible with PHP7?

[SiebelsTim]

This doesn't break any previous versions, does it? Also we have some PHP7 features already in master.

[Majkl578]

Master should be bumped to PHP_VERSION_ID=59999 or something to indicate support for PHP >5.6.

[paulbiss]

I haven't done a feature audit, but we definitely don't have full parity with all PHP 5 versions. That being said we have been adding PHP 7 features along the way. @reeze if you'd like to take a crack at it by all means send us a PR, @andrewparoski would be the best person to talk to about it.

[reeze]

Alright, I will go head and try to implement it.

[FightingMan]

@reeze thumb up

[ptarjan]

@Majkl578 that's a good point. Since we are matching PHP5 (and PHP7) as much as possible on new features we should probably match the constant they are using in their development branch. Although it will be a difficult moving target since we'll say we have PHP7 support in some version (say 3.4), but it won't be complete support since they will surely add more features before their next release that won't be in 3.4. Then if people do feature gating on PHP7 when running HHVM 3.4 it won't work.

What would be best for users?

[staabm]

@ptarjan maybe it would be better to promote feature-testing for hhvm/php compat instead of checking for version numbers

[reeze]

@staabm hmmm but syntax feature is not easy to test.

[Jo-Con-El]

I think it would make more sense to decide whether we want e.g. 80% compatibility and then we flip the switch (i.e. assume that we are able to run it). Or maybe we are not able to say which percentage of compatibility we keep right now. @ptarjan?

[fredemmott]

A concrete percentage doesn't really fit. We've tended to do it when we've implemented all the major new language features and aren't aware of any real-world blockers.

[Jo-Con-El]

Then maybe a blog post or a comparison table? :-)

[fredemmott]

For now, we want compatibility with PHP5. We are generally adding PHP7 features that don't break backwards compatibility. Given PHP7 is nowhere near final, and we're not actively targeting compatibility with it yet, the doesn't seem much value - the comparison would likely be useless by the time there's a stable release of PHP7. If you'd like to continue this discussion, it'd be better on https://www.facebook.com/groups/hhvm.dev/ or as a documentation issue. It's really orthogonal to this issue.

[SiebelsTim]

duplicate #5500

[JoelMarcey]

closing in favor of #5500