add safety for class reset

Author: ben-schwen

R/as.data.table.R

@@ -216,8 +216,9 @@ as.data.table.list = function(x,
 as.data.table.data.frame = function(x, keep.rownames=FALSE, key=NULL, ...) {
   if (is.data.table(x)) return(as.data.table.data.table(x, key=key)) # S3 is weird, #6739. Also # nocov; this is tested in 2302.{2,3}, not sure why it doesn't show up in coverage.
   if (!identical(class(x), "data.frame")) {
+    class_orig = class(x)
     x = as.data.frame(x)
-    setattr(x, "class", "data.frame") # cater for cases when as.data.frame can generate a loop #6874
+    if (identical(class(x), class_orig)) setattr(x, "class", "data.frame") # cater for cases when as.data.frame can generate a loop #6874
     return(as.data.table(x, keep.rownames=keep.rownames, key=key, ...))
   }
   if (!isFALSE(keep.rownames)) {