See rARC3ffed59. I think that test failure is some sort of weird parser generator bug in Bison 3.0.2 that appears to affect Ubuntu and OS X, at a minimum.

I don't immediately see what the acutal issue is, nor can I find anything suspicious in the changelog. However, I can reproduce the issue reliably locally (OS X).

If I do this:

$ git checkout master
$ git clean -df
$ ./scripts/build_xhpast.php

...I get test failures in arc.

If I do this:

$ git checkout bison304 # This change
$ git clean -df
$ ./scripts/build_xhpast.php

...I get clean tests.

The actual AST/parser look fine to me from careful inspection, too.

Added a now-passing unit test.