In the last release I added explicit support for `vector<bool>` which wasn't
working with libc++ because the iterator's `operator*` returned a proxy
reference type. Other implementations return a `bool` for const iterators, so
they happened to work okay.
This solves the problem in a more general purpose way by instantiating
`hash_combine` for the iterator `value_type`. So the type returned by
`operator*` will be implicitly casted to the correct type.
Daniel James