When bit_iterator<It1>::difference_type and bit_iterator<It2>::difference_type can be different?

[viboes]

There are operations such as bit_iterator - bit_iterator that results in common_type of of the difference_types

bit/cpp/bit_iterator.hpp

Line 95 in 67446ab

friend constexpr typename std::common_type<

However the difference_type is not defined explicitly depending on the Iterator.

Does it means that bit_iterator<It> could depend on it It?
Why the wording doesn't require it to be It::diference_type?

[vreverdy]

The only case where difference_type would be different would be user-specialized bit iterators. The wording does not require it to be It::difference_type, because implementations should have the possibility to choose a wider type than std::ptrdiff_t because there are more bits than bytes in memory. Does that answer the question? Do you think that should be different?

[viboes]

I didn't know that the user could specialize those templates. In which cases and how the user can specialize them? I don't see how it could specialize bit_pointer, or bit_iterator<array<unsigned,10>::iterator>?

Do you have a concrete case?