In is.na<-, value can be any index

[danhalligan]

is.na<-.vctrs_vct asserts value argument is a logical.

vctrs/R/type-vctr.R

Line 640 in 69cff44

#' @export

However, in base::R merge.data.frame it may be an index of values to set to NA. See (for example) this section of the merge.data.frame base R function:

        if (all.x) {
            zap <- (lxy + 1L):(lxy + nxx)
            for (i in seq_along(y)) {
                if (is.matrix(y[[1]])) 
                  y[[1]][zap, ] <- NA
                else is.na(y[[i]]) <- zap
            }
        }

[hadley]

From the is.na docs:

(xx <- c(0:4))
#> [1] 0 1 2 3 4
is.na(xx) <- c(2, 4)
xx
#> [1]  0 NA  2 NA  4

^{Created on 2020-03-25 by the reprex package (v0.3.0)}

Also works with character vectors, so we need to treat value vec_as_location().

y <- c(a = 1, b = 2, c = 3)
is.na(y) <- "b"
y
#>  a  b  c 
#>  1 NA  3

^{Created on 2020-03-25 by the reprex package (v0.3.0)}