relax requirements on join wrapper template

Summary: The template that this template wraps requires only forward iterators, so it doesn't make sense for the wrapper to be stricter than that.

Reviewed By: yfeldblum

Differential Revision: D6988987

fbshipit-source-id: 575dcacf7b1873ea0d7112acf84670842ee82fa7

folly/String.h

@@ -534,9 +534,10 @@ std::string join(const Delim& delimiter,
 template <
     class Delim,
     class Iterator,
-    typename std::enable_if<std::is_same<
-        typename std::iterator_traits<Iterator>::iterator_category,
-        std::random_access_iterator_tag>::value>::type* = nullptr>
+    typename std::enable_if<std::is_base_of<
+        std::forward_iterator_tag,
+        typename std::iterator_traits<Iterator>::iterator_category>::value>::
+        type* = nullptr>
 std::string join(const Delim& delimiter, Iterator begin, Iterator end) {
   std::string output;
   join(delimiter, begin, end, output);

folly/test/StringTest.cpp

@@ -21,6 +21,7 @@
 #include <folly/String.h>
 
 #include <cinttypes>
+#include <set>
 
 #include <boost/regex.hpp>
 
@@ -993,6 +994,12 @@ TEST(String, join) {
 
   output = join("", input3.begin(), input3.end());
   EXPECT_EQ(output, "facebook");
+
+  std::multiset<char> input4(input3);
+  output = join("", input4);
+  EXPECT_EQ("abcefkoo", output);
+  output = join("", input4.begin(), input4.end());
+  EXPECT_EQ("abcefkoo", output);
 }
 
 TEST(String, hexlify) {