closes #1681. as.data.table's data.table method returns a copy.

NEWS.md

@@ -195,6 +195,8 @@
 
   55. `fread()` did not respect encoding on header column. Now fixed, [#1680](https://github.com/Rdatatable/data.table/issues/1680). Thanks @nachti.
 
+  56. as.data.table's `data.table` method returns a copy as it should, [#1681](https://github.com/Rdatatable/data.table/issues/1681).
+
 #### NOTES
 
   1. Updated error message on invalid joins to reflect the new `on=` syntax, [#1368](https://github.com/Rdatatable/data.table/issues/1368). Thanks @MichaelChirico.

R/as.data.table.R

@@ -171,8 +171,9 @@ as.data.table.data.frame <- function(x, keep.rownames=FALSE, ...) {
 }
 
 as.data.table.data.table <- function(x, ...) {
+    # as.data.table always returns a copy, automatically takes care of #473
+    x = copy(x) # #1681
     # fix for #1078 and #1128, see .resetclass() for explanation.
     setattr(x, 'class', .resetclass(x, "data.table"))
-    if (!selfrefok(x)) x = alloc.col(x) # fix for #473
     return(x)
 }

inst/tests/tests.Rraw

@@ -8830,6 +8830,12 @@ Encoding(x) = "latin1"
 nm = names(fread("1680-fread-header-encoding.csv", encoding="Latin-1"))
 test(1670, nm[2], x)
 
+# as.data.table must return a copy even if 'x' is a data.table
+x = data.table(a=1, b=2)
+test(1670.1, address(x) != address(as.data.table(x)), TRUE)
+setattr(x, 'class', c('a', class(x)))
+test(1670.2, class(as.data.table(x)), class(x)[2:3])
+
 ##########################
 
 # TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.