S3 class with changeable attribute causes subsetting error

Start by running the minimal MyDate example and compare DT[2, x] with DF[2, 1]. Trace the [.MyDate and [[.MyDate entry points together with data.table column subsetting, then add a regression test showing that the selected value and myattr are preserved. Done means data.table subsetting matches the demonstrated data.frame behavior for this class.

# Minimal reproducible example

Considering this example below. Here a new class named MyDate is created, which inherits from Date and uses an attribute named myattr to store some meta data.

MyDate <- function (x, myattr = 1L) {
    structure(x, myattr = rep_len(myattr, length(x)), class = c("MyDate", "Date"))
}

`[.MyDate` <- function (x, i) {
    r <- NextMethod("[")
    data.table::setattr(r, "myattr", attr(x, "myattr")[i])
    data.table::setattr(r, "class", c("MyDate", "Date"))
    r
}

`[[.MyDate` <- function (x, i) {
    r <- NextMethod("[[", i)
    data.table::setattr(r, "myattr", attr(x, "myattr")[[i]])
    data.table::setattr(r, "class", c("MyDate", "Date"))
    r
}

format.MyDate <- function (x, ...) {
    a <- attr(x, "myattr")
    res <- rep(NA_character_, length(x))
    res[a == 0L] <- "0"
    res[a == 1L] <- as.character(data.table::yday(x[a == 1L]))
    res[a == 2L] <- format.Date(x[a == 2L], "%m-%d")
    res[a == 3L] <- format.Date(x[a == 3L], "%Y-%m-%d")
    res
}

x <- MyDate(0:3, 0:3)
x
# [1] "0"          "2"          "01-03"      "1970-01-04"

x[2]
# [1] "2"
attr(x[2], "myattr")
# [1] 1

x[[2]]
# [1] "2"
attr(x[[2]], "myattr")
# [1] 1

Everything works fine so far. However, if I put x in a data.table, x column cannot be correctly subsetted.

DT <- data.table::data.table(x)
#             x
# 1:          0
# 2:          2
# 3:      01-03
# 4: 1970-01-04
x1 <- DT[2, x]
x1
# [1] "0" NA  NA  NA 
attr(x1, "myattr")
# [1] 0 1 2 3

But data.frame works fine:

DF <- as.data.frame(DT)
DF
#            x
# 1          0
# 2          2
# 3      01-03
# 4 1970-01-04
x2 <- DF[2,1]
x2
# [1] "2"
attr(x2, "myattr")
# [1] 1

# Output of sessionInfo()

R version 3.5.2 (2018-12-20)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Manjaro Linux

Matrix products: default
BLAS: /usr/lib/libblas.so.3.8.0
LAPACK: /usr/lib/liblapack.so.3.8.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=zh_CN.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=zh_CN.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=zh_CN.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=zh_CN.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] nvimcom_0.9-80 colorout_1.2-0

loaded via a namespace (and not attached):
[1] compiler_3.5.2    tools_3.5.2       data.table_1.11.8

Any insight will be greatly appreciated.