added test and the news

Author: venom1204

NEWS.md

@@ -198,6 +198,8 @@ rowwiseDT(
 
 20. Fixed a memory issue causing segfaults in `forder`, [#6797](https://github.com/Rdatatable/data.table/issues/6797). Thanks @dkutner for the report and @MichaelChirico for the fix.
 
+21. `setDT(get0('var'))` now correctly modifies `var` by reference, consistent with the long-standing behavior of `setDT(get('var'))`. Previously, the `.internal.selfref` attribute was not set, [#6864](https://github.com/Rdatatable/data.table/issues/6864). Thanks to @rikivillalba for the report and @venom1204 for the fix.
+
 ### NOTES
 
 1. There is a new vignette on joins! See `vignette("datatable-joins")`. Thanks to Angel Feliz for authoring it! Feedback welcome. This vignette has been highly requested since 2017: [#2181](https://github.com/Rdatatable/data.table/issues/2181).

inst/tests/tests.Rraw

@@ -21183,3 +21183,11 @@ test(2317.6, DT1[DF1, on='a', .(d = x.a + i.d)]$d, 5)
 test(2317.7, DT1[DF2, on='a', e := i.e]$e, 5)
 test(2317.8, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)
 test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
+
+#6864
+ds = copy(datasets::iris)
+x = setDT(get("ds"))
+test(2319.1, !is.null(attr(ds, ".internal.selfref")))
+x = setDT(get0("ds"))
+test(2319.2, !is.null(attr(ds, ".internal.selfref")))
+test(2319.3, identical(ds, x))