allow x. and i. references to tbl in i

Author: MichaelChirico

NEWS.md

@@ -34,6 +34,8 @@
 
 8. `fread()` no longer warns on certain systems on R 4.5.0+ where the file owner can't be resolved, [#6918](https://github.com/Rdatatable/data.table/issues/6918). Thanks @ProfFancyPants for the report and PR.
 
+9. Joins to extended data.frames, e.g. `x[i, col := x.col1 + i.col2]` where `i` is a `tbl`, can use the `x.` and `i.` prefix forms, [#6998](https://github.com/Rdatatable/data.table/issues/6998). Thanks @MichaelChirico for the bug and PR.
+
 ### NOTES
 
 1. Continued work to remove non-API C functions, [#6180](https://github.com/Rdatatable/data.table/issues/6180). Thanks Ivan Krylov for the PRs and for writing a clear and concise guide about the R API: https://aitap.codeberg.page/R-api/.

R/data.table.R

@@ -677,10 +677,12 @@ replace_dot_alias = function(e) {
     }
     ansvals = chmatch(ansvars, nx)
   } else {
-    if (is.data.table(i)) {
+    if (is.data.frame(i)) {
       idotprefix = paste0("i.", names_i)
       xdotprefix = paste0("x.", names_x)
-    } else idotprefix = xdotprefix = character(0L)
+    } else {
+      idotprefix = xdotprefix = character(0L)
+    }
 
     # j was substituted before dealing with i so that := can set allow.cartesian=FALSE (#800) (used above in i logic)
     if (is.null(jsub)) return(NULL)

inst/tests/tests.Rraw

@@ -21133,3 +21133,17 @@ test(2315.1, tail(DT[order(i), i], 2L), 1:2)
 # wider range of numbers needed for further coverage
 DT[1L, i := 1000L]
 test(2315.2, tail(DT[order(i), i], 2L), c(1L, 1000L))
+
+# tbl in i still allows 'i.' prefix reference for update join, #6998
+DT1 = data.table(a=1, b=2)
+DT2 = data.table(a=1, c=3)
+DF1 = data.frame(a=1, d=4)
+DF2 = data.frame(a=1, e=5)
+class(DF2) = c("tbl_df", "tbl", "data.frame")
+
+test(2316.1, DT1[DT2, on='a', c := i.c]$c, 3)
+test(2316.2, DT1[DT2, on='a', c2 := x.a + i.c]$c2, 4)
+test(2316.3, DT1[DF1, on='a', d := i.d]$d, 4)
+test(2316.4, DT1[DF1, on='a', d2 := x.a + i.d]$d2, 5)
+test(2316.5, DT1[DF2, on='a', e := i.e]$e, 5)
+test(2316.6, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)