A post-release update to VS 2017 was the first version which supported a useful subset of C99. But support isn't a binary thing; I was tangentially involved in diagnosing a critical bug in their implementation of C99 lvalue literals just a few months ago. Minimal repro: https://godbolt.org/z/7rTv1M
Minor nitpick: Most C99 features were "already" in a VS2015 update (initialization features like designated init and compound literals, and a standard compliant snprintf()).
The big missing features were VLAs (those will never be implemented) and _Generic (implemented now in VS2019).
Thanks, I was confusing the VS 2015 update with VS 2017 as the one that got the big pieces like designated initializers and compound literals. I don't think anyone cares about VLAs, but I look forward to being able to rely on _Generic in another 5 years when VS 2019 can be assumed available. :)
