Consider vec_proxy() and vec_restore() methods for IDate

[DavisVaughan]

From tidyverse/dplyr#6687

library(vctrs)
library(data.table)

x <- as.IDate("2017-06-05")
class(x)
#> [1] "IDate" "Date"

typeof(x)
#> [1] "integer"

typeof(vec_slice(x, 1))
#> [1] "double"

# Because of
typeof(vec_proxy(x))
#> [1] "double"

# Proxy is inherited from Date, which normalizes to double
vctrs:::vec_proxy.Date
#> function (x, ...) 
#> {
#>     date_validate(x)
#> }
#> <bytecode: 0x7fedebbeb8f0>
#> <environment: namespace:vctrs>

[buggaby]

I'm seeing this occur with a filter call on a data.table. Is this the same issue?

Dates = as.IDate(c("2018-04-02", "2018-04-03", "2018-04-04"))
Letters = c("A", "B", "C")
dt = data.table(Date = Dates, Letter = Letters)

dt %>%
	pull(Date) %>%
	typeof()
#> [1] "integer"

dt %>%
	filter(Letter != "A") %>%
	pull(Date) %>%
	typeof()
#> [1] "double"