After Visual Studio 2017 was released I wanted to try to create simple console project with new C# 7 features. I expected that I simply download new Visual Studio 2017, then
Vs2017 update 3 can support c#7.1 but it's configured by default to support c#7.0.
You can modify the setting of your project and select c# 7.1
for more details how to configure vs2017.3 to support last version of c#7.1