The separately compiled parts of the C++ Standard Library have shipped in VS installations for many years (look for xrngdev.cpp in a subdirectory typically named crt/src, that should be distinctive enough). This was intended for debugging purposes, covered by the VS EULA as always. These parts weren’t standalone-buildable and we occasionally forgot to ship a file or two, but we always tried to make it all source-readable.

Now, being open source enables much more.