${#VAR:-foo} (or any other modifier on ${#VAR} is a syntax error.
On the other hand ${##} is not, nor is ${##13} though they mean
quite different things (the latter is an idiom everyone should learn,
... $# except we refuse to admit the possibility that it is 13...
Even I cannot explain what ${#-foo} used to do, but it wasn't sane!
(It should be just $# as $# is never unset, but ...)
Shell syntax is truly a wondrous thing!
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