Major Description
A post on the solr-user mailing list drew my attention to the fact that this is apparently a valid range query under the QueryParser.jj grammer (both for the classic parser and the solr variant – i didn't check flexible)...
[x z] // parsed the same as [x TO z]
it's parsed as a valid range query even though there is no {{ TO }} – even though there is nothing in the docs to suggest that the {{ TO }} is intended to be optional.
Furthermore, in my experimenting i realized that how the grammer looks for the {{ TO }} keyword seems to be a bit sloppy, making some range queries that should be easy to validate (because they are unambiguous) fail to parse...
[TO TO z] // fails [a TO TO] // fails [a TO "TO"] // works, but why should quoting be neccessary here?
If the "sloppy" parsing behavior is intentional, then the docs should reflect that the {{ TO }} is optional – but it seems like in general we should make these other unambiguous cases parse cleanly.