When using arc with PHP 7.1, and I attempt to run arc liberate on the Phabricator repository (e.g. because I wrote a patch), the process will mysteriously crash due to a segmentation fault in the underlying xhpast binary (I assume), causing the overall liberation process to fail spectacularly. This happens when xhpast is being used to enumerate and process the symbols/class dependencies of the PHP code, so that e.g. arc lint can tell you "hey, I can't resolve that symbol", etc.
I strongly believe this is a PHP 7.1-related flaw, though I have not investigated completely. The reason I think this is because I was able to reproduce it on two completely separate Linux systems (Fedora 25 and Ubuntu) with radically different toolchains/environments, yet I was able to concretely reproduce the failure on both of them. I have never seen this on any PHP 5.x related build, even on recent Fedora/Ubuntu machines (I only recently started using PHP 7.1 on my brand-new laptop).
Note this is made more complex by the fact the segmentation fault occurs seemingly randomly; it does not always fault on the same file, but it always eventually faults, with a probability of 1, on some random file. Even if I run the xhpast subcommand that crashed on a specific Phabricator file like 1,000 times repeatedly in a loop, I never get a failure twice. But I can always cause it to crash, no matter what, if I just run it and wait. Sometimes arc liberate fails 10% in, other times 20%, etc.
I'm reporting this from my desktop so I'll attach some traces and more information when I move over to my laptop later.