I don't know anything about Chapel beyond what's on this page, but while I'd guess that you're correct, that where clauses only work with constants, given Chapel's widespread use of range types, it's possible that it might be able to prove when numbers are withing a certain range. A lot of languages do this to avoid inserting bounds-checking code to array access where a number can be trivially shown to always be within range. Once you start doing interesting things with numbers though, your ranges start to become to wide to be useful.