Why can't the command-line csc compile C# 7 after installing VS 2017?

Question

I just downloaded Visual Studio 2017 RC, which was released a few days ago and comes with support for C# 7. I can use C# 7 features from the IDE:

Answer 1

You have to launch "Developer Command Prompt for VS 2017 RC". Then you can see that csc.exe will have a version number of 2.0.

It has been well known that for every new VS release you should use its dedicated prompt with the appropriate environment variables loaded.