"circular_buffer":"BlockDiagram::Filter<T>StampedValue" is not a valid template type argument for parameter "T"

Question

Following implementation leads to the error "circular_buffer":"BlockDiagram::FilterStampedValue" is not a valid template type argument for parameter "T".

Header-File:

namespace BlockDiagram {
  template<class T=double>
  class Filter {
   public:
    typedef struct {
        std::chrono::time_point<std::chrono::system_clock> tv;
        T  d;
    } StampedValue;
    typedef circular_buffer<Filter<T>::StampedValue> cbuf_type;...}

Cpp-File:

namespace BlockDiagram {

template<class T>
Filter<T>::Filter() {
    ...
}}

Unfortunately I have no clue what could be wrong in this implementation.