This is something I came across just now:
f <- function(x) missing(x)
z <- quote(expr=)
f(z)
# TRUE
The object z contains the equivalent of a missing function argument. Another method for generating a missing arg would be alist(a=)$a .
Should f(z) return TRUE in this case? I interpret missing() as checking whether the parent function call had a value supplied for the given argument. Here, I have supplied an argument (z), so I would expect f to return FALSE.