C++ Templates nested access

I just learned something cool about C++ templates you can set constraints based on the internals of a template parameter

Inside size_t Size = Point_::Size is looking at the type defined inside the Point_ type (which can be anything) and using it to define the types of the Matrix.

template <typename Point_, bool Affine>
struct Transform {
    // =============================================================
    //! @{ \name Type declarations
    // =============================================================

    static constexpr size_t Size = Point_::Size;
    static constexpr bool IsAffine = Affine;

    using Float   = dr::value_t<Point_>;
    using Matrix  = dr::Matrix<Float, Size>;
}

This snippet comes from the Mitsuba3 codebase