If this doesn't work out, maybe we can try one of these:
a. fall back to bare file name if expression doesn't contain any operator-like
characters (e.g. "f(x" is an error, but "f x" can be parsed as bare string)
b. introduce command-line flag to opt in (e.g. -e FILESET)
c. introduce pattern prefix to opt in (e.g. set:FILESET)
Closes#3239, #2915, #2286