Cannot able to get private member behind base class

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example:

using VD = std::variant<double, std::string_view>;
template struct access_private::access<&VD::_M_index>; 

... 

VD vd;
accessor<"_M_index">(vd); // fail, because this member is hidden behind private base class

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Currently, it can be a workaround if you know the exact type (#10 if you don't):

• If the base class is public, you can query the accessor with the base class.

• If the base class is private:

  • Create a private base conversion (clang, or non-constexpr way on pointer interconvertible base)
  • Cast member pointer to the derived class's member pointer. (only clang, GCC bug)