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### 3385. `common_iterator` is not sufficiently constrained for non-copyable iterators

Section: 25.5.5.1 [common.iterator] Status: C++20 Submitter: Corentin Jabot Opened: 2020-01-31 Last modified: 2021-02-25

Priority: 0

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Discussion:

We don't actually specify anywhere that non-copyable iterators do not have a `common_iterator` (and it would make little sense for them to as algorithms dealing with C++17 iterators are not expecting non-copyable things) As it stands `common_iterator` can be created from move only iterator but are then non-copyable themselves. P1862 already constrains `common_view` in a similar fashion

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.2 [iterator.synopsis], header `<iterator>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 25.5.5 [iterators.common], common iterators
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >           class common_iterator;
   >       […]
   >     }

2. Modify 25.5.5.1 [common.iterator], class template `common_iterator` synopsis, as indicated:

   >     namespace std {
   >       template<input_or_output_iterator I, sentinel_for<I> S>
   >         requires (!same_as<I, S> && copyable<I>)
   >       class common_iterator {
   >       public:
   >         […]
   >       };
   >       […]
   >     }