Fix index printing by adding index info to header (#6816)

Produce the class header from `toprint` instead of just `x`. Fixes: #6806 

Co-authored-by: Ivan K <[email protected]>

Author: Mukulyadav2004

R/print.data.table.R

@@ -86,6 +86,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     if (show.indices) toprint = cbind(toprint, index_dt)
   }
   require_bit64_if_needed(x)
+  classes = classes1(toprint)
   toprint=format.data.table(toprint, na.encode=FALSE, timezone = timezone, ...)  # na.encode=FALSE so that NA in character cols print as <NA>
 
   # FR #353 - add row.names = logical argument to print.data.table
@@ -100,7 +101,6 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
       factor = "<fctr>", POSIXct = "<POSc>", logical = "<lgcl>",
       IDate = "<IDat>", integer64 = "<i64>", raw = "<raw>",
       expression = "<expr>", ordered = "<ord>")
-    classes = classes1(x)
     abbs = unname(class_abb[classes])
     if ( length(idx <- which(is.na(abbs))) ) abbs[idx] = paste0("<", classes[idx], ">")
     toprint = rbind(abbs, toprint)

inst/tests/tests.Rraw

@@ -21069,3 +21069,9 @@ if (exists("sort_by", "package:base")) {
   test(2306.23, DT1[, sort_by(.SD, ~a + b)], data.table(a=c(1, 2, 3, 3, NA), b=c(4L, 2L, 0L, 3L, 1L)))
   test(2306.24, DT1[, sort_by(.SD, ~.(a, b))], data.table(a=c(1, 2, 3, 3, NA), b=c(4L, 2L, 0L, 3L, 1L)))
 }
+
+DT <- data.table(a = 1:2, b = 2:1)
+setindex(DT, b)
+# make sure that print(DT) doesn't warn due to the header missing index column types, #6806
+# can't use output= here because the print() call is outside withCallingHandlers(...)
+test(2307, { capture.output(print(DT, class = TRUE, show.indices = TRUE)); TRUE })