explicit format.tbl_df() method, closes #263

- The `format.tbl_df()` method is identical to `format.tbl()`, instead of calling `NextMethod()`, to protect against malformed tibbles that inherit from `"tbl_df"` but not `"tbl"`, as created e.g. by `ungroup()` in dplyr 0.5.0 and earlier (#263).

NAMESPACE

@@ -28,6 +28,7 @@ S3method(check_names_df,default)
 S3method(check_names_df,logical)
 S3method(check_names_df,numeric)
 S3method(format,tbl)
+S3method(format,tbl_df)
 S3method(format,trunc_mat)
 S3method(format_v,character)
 S3method(format_v,default)

R/tbl-df.r

@@ -14,6 +14,10 @@ format.tbl <- function(x, ..., n = NULL, width = NULL, n_extra = NULL) {
   format(trunc_mat(x, n = n, width = width, n_extra = n_extra))
 }
 
+#' @rdname formatting
+#' @export
+format.tbl_df <- format.tbl
+
 #' @rdname formatting
 #' @export
 print.tbl <- function(x, ..., n = NULL, width = NULL, n_extra = NULL) {