Interesting that their implementation is Apache 2.0 licensed, yet includes exceptions for LLVM and for GPLv2 licensed code/projects wrt patents.
Does anyone know if the library's quality is on par with the GNU or Clang libraries? Google has their own too, if memory serves. Is there an implementation deemed "the best"?
The license was explicitly chosen to enable code sharing with LLVM's libc++ (https://devblogs.microsoft.com/cppblog/open-sourcing-msvcs-s...).
The MSVC STL's quality is good enough for thousands of pieces of Windows software (including Windows itself & Microsoft's software such as Office) to depend and rely on. It delivers excellent performance for a broad range of use cases. It is actively developed in the open, delivering cutting-edge (C++23 & C++26) features, accepting Pull Requests and wonderfully documented on GitHub. It can be consumed using MSVC and LLVM clang-cl (which the MSVC STL maintainers test with CI infrastructure). The maintainers are actively working on "hardening" features to enable more secure C++ (https://github.com/microsoft/STL/wiki/STL-Hardening).
Unless you specify what "best" or "a library's quality" means to you, MSVC STL is excellent and because of that, the default choice on & for Windows.
Google chooses to only support libc++ for Chrome/Chromium (https://chromium.googlesource.com/chromium/src/+/main/docs/t...). libc++ is not a Google-owned project.