Merge pull request #1679 from MichaelChirico/dimnames

Closes #1678; dimnames.data.table properly uses inherits to check data.frame awareness

NEWS.md

@@ -190,6 +190,8 @@
   53. `fread` won't use `wget` for file:// input, [#1668](https://github.com/Rdatatable/data.table/issues/1668); thanks @MichaelChirico for FR&PR.
 
   54. `chmatch()` handles `nomatch = integer(0)` properly, [#1672](https://github.com/Rdatatable/data.table/issues/1672).
+
+  55. `dimnames.data.table` no longer errors in `data.table`-unaware environments when a `data.table` has, e.g., been churned through some `dplyr` functions and acquired extra classes, [#1678](https://github.com/Rdatatable/data.table/issues/1678). Thanks Daisy Lee on SO for pointing this out and @MichaelChirico for the fix.
 
   55. `fread()` did not respect encoding on header column. Now fixed, [#1680](https://github.com/Rdatatable/data.table/issues/1680). Thanks @nachti.
 

R/data.table.R

@@ -2022,7 +2022,8 @@ as.list.data.table <- function(x, ...) {
 
 dimnames.data.table <- function(x) {
     if (!cedta()) {
-        if (!identical(class(x),c("data.table","data.frame"))) stop("data.table inherits from data.frame (from v1.5) but this data.table does not. Has it been created manually (e.g. by using 'structure' rather than 'data.table') or saved to disk using a prior version of data.table? The correct class is c('data.table','data.frame').")
+        if (!inherits(x, "data.frame")) 
+          stop("data.table inherits from data.frame (from v1.5), but this data.table does not. Has it been created manually (e.g. by using 'structure' rather than 'data.table') or saved to disk using a prior version of data.table?")
         return(`dimnames.data.frame`(x))
     }
     list(NULL, names(x))