Variables in wrong class

Just found out with my clang static source analyzer: We have

class Z4::AbstractZ4Solver: public exahype::solvers::ADERDGSolver {
  public:
    static constexpr int NumberOfVariables  = 54;
    static constexpr int NumberOfParameters = 0;
    static constexpr int Order              = 3;
  
    class Variables;
    class ReadOnlyVariables;
    class Fluxes;
...

but then

class Z4::Z4Solver::ReadOnlyVariables {
  private:
    const double* const _Q;
  public:
    static constexpr int SizeVariables  
...

However, it should be Z4::AbstractZ4Solver::ReadOnlyVariables or the forward declarations have to change classes.