[const.iterators.alias]

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template<indirectly_readable It>
  using iter_const_reference_t =
    common_reference_t<const iter_value_t<It>&, iter_reference_t<It>>;

template<class It>
  concept constant-iterator =                                                   / exposition only
    input_iterator<It> && same_as<iter_const_reference_t<It>, iter_reference_t<It>>;

template<input_iterator I>
  using const_iterator = see below;

Result: If I models constant-iterator, I.

Otherwise, basic_const_iterator<I>.

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template<semiregular S>
  using const_sentinel = see below;

Result: If S models input_iterator, const_iterator<S>.

Otherwise, S.

24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.3 Constant iterators and sentinels [const.iterators]

24.5.3.2 Alias templates [const.iterators.alias]