Which "C# Experimental language feature" is this? -

in example below, resharper shows "c# experimental language feature" tooltip on first curly bracket. i've checked new features of c# 6.0 didn't come across similar one. referred experimental feature?

 class class1  {      { // <= c# experimental language feature      }  } 

note: error .net framework 4.5 compiler.

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

this was primary constructors, feature has been cut c#6.