Function assign_expr_to_initialized

include/win/boost/optional/optional.hpp:559–563

    template<class Expr>
    void assign_expr_to_initialized ( Expr&& factory, in_place_factory_base const* tag )
     {
       destroy();
       construct(factory,tag);
     }

    // Constructs in-place using the given typed factory
    template<class Expr>