strange error with interface to parameterized derived type

[NightStrike]

module a
    implicit none
    type :: T(n)
        integer, len :: n
        real :: r
    end type

    interface
        subroutine sub(x)
                import T
                type(T(*)) :: x
        end subroutine
    end interface
end module

$ gfortran -c a.f90
a.f90:11:26:

                 type(T(*)) :: x
                          1
Error: Derived type 'pdtt' at (1) is being used before it is defined


Where does the prefix "pdt" come from?  And should the above code work?