Today at CppCon 2019, the MSVC team announced that they are releasing their implementation of the
C++ Standard Library (also known as the STL) as open source.

The new --> repository <-- contains all of the product source code, a new CMake build system, and a README with more information.

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А раньше она чем была? Это ж библиотека шаблонов, она только в исходниках и может быть.

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rmn пишет:
А раньше она чем была?

да тем же самым и была.
Тут смысл не в том, что теперь это open source as oppose to closed private, а скорее в github'e.
Может название не совсем удачное.
вот что говорят сами авторы:

Q: Why are you doing this?

A: There are several reasons.
Working on the STL in GitHub will allow our customers to follow our development as it happens,
try out our latest changes, and help improve our pull requests by reviewing them.
As C++ Standardization accelerates, with more large features being voted in every year,
we believe that accepting major features as open source contributions will be important.
(For example, C++20’s chrono and format libraries are potential candidates.)
We also want to contribute back to the C++ community by making it possible to take our implementations of major features. (For example, C++17’s charconv.)

надеюсь это прояснило ситуацию?

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