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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:
       […]
     };
     […]
   }