Which “C# Experimental language feature” is this?

Question

In the example below, Resharper shows "C# Experimental language feature" tooltip on the first curly bracket. I've checked the new features of C# 6.0 but didn't come across a similar one. What is the referred experimental feature?

 class Class1
 {
     { // <= C# Experimental language feature
     }
 }

Note: It is an error for .Net Framework 4.5 compiler.

"Invalid token '{' in class, struct, or interface member declaration"

Answer 1

This was for Primary Constructors, a feature which has now been cut from C#6.

Answer 2

On November 12, 2014 Microsoft announced the release of Visual Studio 2015 on the day of Visual Studio Connect() from New York, USA where they announced many new features as well as enhancements with C# 6.0. So we should understand that a Primary Constructor is not applicable with C# 6.0 but we can do the same thing using Auto Implemented Properties Initializers.

Basically a Primary Constructor is a feature of C# that was announced with Visual Studio 2014. But now it has been removed from Visual Studio 2015 Preview. So in C# 6.0 it's not possible to use a primary constructor.

Answer 3

You can still use this if you click on your project, and go to the Properties pane and select C# Language Level to Experimental.