fix distinct for zero columns data frames. closes tidyverse#1437

NEWS.md

@@ -21,6 +21,8 @@
 
 * Faster joining by character column (#1386).
 
+* Fixed `distinct` for zero column data frames (#1437). 
+
 # dplyr 0.4.3
 
 ## Improved encoding support

src/distinct.cpp

@@ -5,6 +5,8 @@ using namespace dplyr ;
 
 // [[Rcpp::export]]
 SEXP distinct_impl( DataFrame df, CharacterVector vars){
+    if( df.size() == 0 )
+        return df ;
     check_valid_colnames(df) ;
     if( !vars.size() ){
         vars = df.names() ;

tests/testthat/test-distinct.R

@@ -23,3 +23,9 @@ test_that("grouped_by uses grouping vars & preserves groups", {
   compare_tbls(tbls[c("df", "dt")],
     function(x) x %>% group_by(x) %>% distinct(y))
 })
+
+test_that("distinct works for 0-sized columns (#1437)", {
+  df <- data_frame(x=1:10) %>% select(-x)
+  ddf <- distinct(df)
+  expect_equal( ncol(ddf), 0L )
+})