A snippet from the paper that made me understand what this is about:

  // $ is the reflection operator and T is a type.
  constexpr { // execute this at compile time
   for... (auto m : $T.variables()) // examine each member variable m in T
     if (m.name() == “xyzzy”) // if there is one with name “xyzzy”
       -> { int plugh; } // then inject also an int named “plugh”
  }

This would inject "int plugh;" in the scope surrounding the constexpr block.