Use %iscall% to handle more general by= inputs correctly (#6499)

Author: MichaelChirico

NEWS.md

@@ -69,6 +69,8 @@ rowwiseDT(
 
 11. `fread()` automatically detects timestamps with sub-second accuracy again, [#6440](https://github.com/Rdatatable/data.table/issues/6440). This was a regression due to interference with new `dec='auto'` support. Thanks @kav2k for the concise report and @MichaelChirico for the fix.
 
+12. Using a namespace-qualified call on the RHS of `by=`, e.g. `DT[,.N,by=base::mget(v)]`, works again, fixing [#6493](https://github.com/Rdatatable/data.table/issues/6493). Thanks to @mmoisse for the report and @MichaelChirico for the fix.
+
 ## NOTES
 
 1. Tests run again when some Suggests packages are missing, [#6411](https://github.com/Rdatatable/data.table/issues/6411). Thanks @aadler for the note and @MichaelChirico for the fix.

R/data.table.R

@@ -789,7 +789,7 @@ replace_dot_alias = function(e) {
           tt = eval(bysub, parent.frame(), parent.frame())
           if (!is.character(tt)) stopf("by=c(...), key(...) or names(...) must evaluate to 'character'")
           bysub=tt
-        } else if (is.call(bysub) && !(bysub[[1L]] %chin% c("list", "as.list", "{", ".", ":"))) {
+        } else if (is.call(bysub) && !(bysub %iscall% c("list", "as.list", "{", ".", ":"))) {
           # potential use of function, ex: by=month(date). catch it and wrap with "(", because we need to set "bysameorder" to FALSE as we don't know if the function will return ordered results just because "date" is ordered. Fixes #2670.
           bysub = as.call(c(as.name('('), list(bysub)))
           bysubl = as.list.default(bysub)

inst/tests/tests.Rraw

@@ -19263,3 +19263,8 @@ df = data.frame(a=1:3)
 setDT(df)
 attr(df, "att") = 1
 test(2291.1, set(df, NULL, "new", "new"), error="attributes .* have been reassigned")
+
+# ns-qualified bysub error, #6493
+DT = data.table(a = 1)
+test(2292.1, DT[, .N, by=base::mget("a")], data.table(a = 1, N = 1L))
+test(2292.2, DT[, .N, by=base::c("a")], data.table(a = 1, N = 1L))