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<title>Issue 3989: The whole range for an iterator obtained from a std::span or std::basic_string_view is not clear</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="3989"><a href="lwg-active.html#3989">3989</a>. The whole range for an iterator obtained from a <code>std::span</code> or <code>std::basic_string_view</code> is not clear</h3>
<p><b>Section:</b> 27.3 <a href="https://wg21.link/string.view">[string.view]</a>, 23.7.2.2 <a href="https://wg21.link/views.span">[views.span]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-08-29 <b>Last modified:</b> 2023-10-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#string.view">active issues</a> in [string.view].</p>
<p><b>View all other</b> <a href="lwg-index.html#string.view">issues</a> in [string.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It is unclear whether the following program has undefined behavior:
</p>
<blockquote><pre>
#include &lt;cassert&gt;
#include &lt;span&gt;
#include &lt;string_view&gt;

int main()
{
  int arr[2]{42, 84};
  std::span&lt;int&gt; sp1{arr, 1};
  std::span&lt;int&gt; sp2{arr + 1, 1};

  assert(sp2.begin() - sp1.begin() == 1); // Is this well-defined?
  assert(sp2.begin() == sp1.end());       // ditto
  assert(*sp1.end() == 84);               // ditto

  const char str[]{"string"};
  std::string_view sv1{str, 3};
  std::string_view sv2{str + 3, 3};

  assert(sv2.begin() - sv1.begin() == 3); // Is this well-defined?
  assert(sv2.begin() == sv1.end());       // ditto
  assert(*sv1.end() == 'i');              // ditto
}
</pre></blockquote>
<p>
Currently MSVC STL strictly diagnose the arithmetic between different <code>span</code>s/<code>basic_string_view</code>s, 
even the views are on the same underlying range (see <a href="https://github.com/microsoft/STL/issues/1435">this Github issue</a>). 
libstdc++ and libc++ silently accept these operations.
<p/>
Perhaps the standard should clarify what the whole ranges are when determining whether operations between iterators 
from different views on the same underlying range is well-defined.
</p>

<p><i>[2023-10-30; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Libc++ diagnoses the example with the right macros defined.
"Should <code>substr</code> and <code>remove_suffix</code> tighten the
bounds or copy them from the original view?"
</p>



<p id="res-3989"><b>Proposed resolution:</b></p>





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