Mark BitIterator with the category of the base

Summary:
[Folly] Mark `BitIterator<BaseIterator>` with the iterator category of `BitIterator`.

As one benefit, using `std::distance` to find the distance between two `BitIterator`s gotten from a random access container such as `std::vector` is now constant-time.

Fixes #1026.

Reviewed By: ot

Differential Revision: D14294852

fbshipit-source-id: 2345bc73dec169803ae41b9391687e89ad77207b

folly/container/detail/BitIteratorDetail.h

@@ -80,7 +80,8 @@ struct BitIteratorBase {
       BitIterator<BaseIter>, // Derived
       BaseIter, // Base
       bool, // Value
-      boost::use_default, // CategoryOrTraversal
+      typename std::iterator_traits<
+          BaseIter>::iterator_category, // CategoryOrTraversal
       bititerator_detail::BitReference<
           typename std::iterator_traits<BaseIter>::reference,
           typename std::iterator_traits<BaseIter>::value_type>, // Reference

folly/container/test/BitIteratorTest.cpp

@@ -16,7 +16,9 @@
 
 #include <folly/container/BitIterator.h>
 
+#include <forward_list>
 #include <limits>
+#include <list>
 #include <type_traits>
 #include <vector>
 
@@ -84,3 +86,25 @@ TEST(BitIterator, Const) {
   checkIt(0x10, bi);
   checkIt(0x42, bi);
 }
+
+TEST(BitIterator, IteratorCategory) {
+  EXPECT_TRUE( //
+      (std::is_same<
+          std::iterator_traits<BitIterator<uint64_t*>>::iterator_category,
+          std::random_access_iterator_tag>::value));
+  EXPECT_TRUE(
+      (std::is_same<
+          std::iterator_traits<
+              BitIterator<std::vector<uint64_t>::iterator>>::iterator_category,
+          std::random_access_iterator_tag>::value));
+  EXPECT_TRUE(
+      (std::is_same<
+          std::iterator_traits<
+              BitIterator<std::list<uint64_t>::iterator>>::iterator_category,
+          std::bidirectional_iterator_tag>::value));
+  EXPECT_TRUE( //
+      (std::is_same<
+          std::iterator_traits<BitIterator<
+              std::forward_list<uint64_t>::iterator>>::iterator_category,
+          std::forward_iterator_tag>::value));
+}