Just announced at CppCon:

Open Sourcing MSVC's STL

From the announcement:

Today at CppCon 2019, we (the MSVC team) announced that we’re releasing our implementation of the C++ Standard Library (also known as the STL) as open source.

https://github.com/microsoft/STL is our new repository, containing all of our product source code, a new CMake build system, and a README with more information. As it explains, we’re still working on migrating to GitHub. While you can clone and build the code right now, there are several things we need to do (like adding our test suites) before we can begin reviewing pull requests.

Q&A

Q: Why are you doing this?

A: There are several reasons. ...