Problem with colnames in derived matrix from data.table comparison

[eyherabh]

The comparison of two data.tables yields a matrix for which the value returned by colnames() changes when columns are added by reference to the leftmost data.table which the matrix was created from. Specifically, consider that a <- b <- data.table(X=1:2). Then, x <- a==b; print(x) returns a matrix

        X
[1,] TRUE
[2,] TRUE

and colnames(x) returns "X" as expected. Adding the column Y as a[, Y:=1] results in colnames(x) returning print(x)returning"X" "Y", even though print(x)still returns the same as above. Further, this does not happen if the column is added asa$Y <- 1` or if the column is added to b instead. I have not managed to reproduce the problem with data.frames, thus the chances are this is a problem that can be solved here.

# Minimal reproducible example

> ## The section below reproduces the problem
> a <- b <- data.table(X=1:2)
> x <- a==b
> print(x)
        X
[1,] TRUE
[2,] TRUE
> colnames(x)
[1] "X"
> 
> ## Adding a column to a by reference causes the column names of x to change.
> a[, Y:=1]
> print(x)
        X
[1,] TRUE
[2,] TRUE
> colnames(x)
[1] "X" "Y"
>

# Output of sessionInfo()
R version 3.6.2 (2019-12-12)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.6 LTS

Matrix products: default
BLAS: /usr/lib/openblas-base/libblas.so.3
LAPACK: /usr/lib/libopenblasp-r0.2.18.so

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

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

other attached packages:
[1] data.table_1.12.8

loaded via a namespace (and not attached):
[1] compiler_3.6.2