Bug: Rehashing an empty unordered_set with cache_begin set to true hits an assert

[seelabs]

Creating a boost::intrusive::make_unordered_set with cache_begin set to true and immediately rehashing will hit an assert. The assert that's hit is here:

intrusive/include/boost/intrusive/hashtable.hpp

Line 1264 in 5c4d116

BOOST_INTRUSIVE_INVARIANT_ASSERT(insertion_bucket < this->bucket_hash_type::priv_bucket_count());

If cache_begin is set to false, or the collection is not empty, the assert is not hit.

Here is a gist of a small program that will reproduce the issue: https://gist.github.com/seelabs/c91655d1a3516f854b17145faac7db25

[igaztanaga]

Many thanks for the report and the test case. And sorry for the delay replying.