Allow setDT(get(...)) to work as previously (#6726)

* Allow setDT(get(...)) to work as previously

* Quirks of test.data.table...

* need to eval() in the right place

* imitate the approach in other branches more closely

* maybe we just needed enclos=?

* Comment for posterity

* Actually do the test

R/data.table.R

@@ -2987,6 +2987,12 @@ setDT = function(x, keep.rownames=FALSE, key=NULL, check.names=FALSE) {
     } else if (isS4(k)) {
       .Call(CsetS4elt, k, as.character(name[[3L]]), x)
     }
+  } else if (name %iscall% "get") { # #6725
+    # edit 'get(nm, env)' call to be 'assign(nm, x, envir=env)'
+    name = match.call(get, name)
+    name[[1L]] = quote(assign)
+    name$value = x
+    eval(name, parent.frame(), parent.frame())
   }
   .Call(CexpandAltRep, x)  # issue#2866 and PR#2882
   invisible(x)

inst/tests/tests.Rraw

@@ -20653,6 +20653,18 @@ setDT(d2)
 test(2295.1, !is.data.table(d1))
 test(2295.2, rownames(d1), 'b')
 test(2295.3, is.data.table(d2))
+# Ensure against regression noted in #6725
+x = data.frame(a=1)
+e = environment()
+foo = function(nm, env) {
+  setDT(get(nm, envir=env))
+}
+foo('x', e)
+test(2295.4, is.data.table(x))
+e = new.env(parent=topenv())
+e$x = data.frame(a=1)
+foo('x', e)
+test(2295.5, is.data.table(e$x))
 
 # #6588: .checkTypos used to give arbitrary strings to stopf as the first argument
 test(2296, d2[x %no such operator% 1], error = '%no such operator%')