[boost] [license] Boost license question

If some compiler vendor takes a version of Boost and makes changes to that version of Boost to suit the particular compiler, while distributing that version with full source code as an optional part of some release of that compiler, does this mean that the code is now proprietary for that compiler vendor ? Or does the code still remain open source so that a programmer can use the changes made to update his own version of a Boost release or provide PRs to some later release of Boost based on the code in the compiler vendor's release ?