Fix USE_STORED_HASH_ON_REHASH to return true when bucket_count is 0, …

…STORE_HASH is true and is_power_of_two_policy<GrowthPolicy>::value is true

This commit doesn't change the actual behaviour of the map as even when USE_STORED_HASH_ON_REHASH was returning false on empty map rehashes, no stored hashes were used as the map was empty.

include/tsl/robin_hash.h

@@ -410,9 +410,9 @@ class robin_hash : private Hash, private KeyEqual, private GrowthPolicy {
       TSL_RH_UNUSED(bucket_count);
       return true;
     } else if (STORE_HASH && is_power_of_two_policy<GrowthPolicy>::value) {
-      tsl_rh_assert(bucket_count > 0);
-      return (bucket_count - 1) <=
-             std::numeric_limits<truncated_hash_type>::max();
+      return bucket_count == 0 ||
+             (bucket_count - 1) <=
+                 std::numeric_limits<truncated_hash_type>::max();
     } else {
       TSL_RH_UNUSED(bucket_count);
       return false;

tests/robin_map_tests.cpp

@@ -140,6 +140,14 @@ BOOST_AUTO_TEST_CASE(test_range_insert) {
   }
 }
 
+BOOST_AUTO_TEST_CASE(test_rehash_0) {
+  tsl::robin_map<int, int, std::hash<int>,
+                   std::equal_to<int>,
+                   std::allocator<std::pair<int, int>>,
+                   true> map;
+  map.rehash(0);
+}
+
 BOOST_AUTO_TEST_CASE(test_insert_with_hint) {
   tsl::robin_map<int, int> map{{1, 0}, {2, 1}, {3, 2}};