[expr.prim.lambda.capture]/12:
An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy. It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference. If declared, such non-static data members shall be of literal type.
The closure types have direct access to objects, so why are the reference members sometimes needed? It even only requires the members to be of literal type, why?