Is std::ranges::size supposed to return an unsigned integer?

Here it is written that std::ranges::size should return an unsigned integer. However, when I use it on an Eigen vector (with Eigen 3.4) the following compiles:

Eigen::VectorXd x;
static_assert(std::same_as<Eigen::VectorXd::Index,
                           decltype(std::ranges::size(x))>);

where Eigen::VectorXd::Index is notoriously a signed integer. By looking at the implementation of std::ranges::size, I noticed that the return type is inferred from the return type of x.size(), which is precisely Eigen::VectorXd::Index. Is this a bug of std::ranges::size? Or is this expected?

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