[license] Boost license question

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-01-23 19:02:51

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 ?