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<title>Issue 3817: Missing preconditions on forward_list modifiers</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#C++23">C++23</a> status.</em></p>
<h3 id="3817"><a href="lwg-defects.html#3817">3817</a>. Missing preconditions on <code>forward_list</code> modifiers</h3>
<p><b>Section:</b> 23.3.7.5 <a href="https://wg21.link/forward.list.modifiers">[forward.list.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tomasz Kamiński <b>Opened:</b> 2022-11-08 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#forward.list.modifiers">issues</a> in [forward.list.modifiers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This is resolution of <b><a href="https://github.com/cplusplus/nbballot/issues/456">GB-101</a></b>
(23.3.7.5 <a href="https://wg21.link/forward.list.modifiers">[forward.list.modifiers]</a> p12,15,20,21 Missing preconditions on forward_list modifiers).
</p>

<p>
Some of the modifiers to <code>forward_list</code> are special to that container and accordingly are not described 
in 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>. Specifically, <code>insert_after</code> (iterator overload), 
<code>insert_range_after</code> and <code>emplace_after</code> do not verify that the <code>value_type</code> is <i>Cpp17EmplaceConstructible</i>
from the appropriate argument(s).
Furthermore <code>insert_after</code> (value overloads) are missing <i>Cpp17CopyInsertable</i>/<i>Cpp17MoveInsertable</i> 
requirements.
</p>

<p><i>[Kona 2022-11-08; Move to Ready]</i></p>

<p><i>[Kona 2022-11-12; Correct status to Immediate]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3817"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>

<li><p>Modify 23.3.7.5 <a href="https://wg21.link/forward.list.modifiers">[forward.list.modifiers]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: <code>emplace_front</code>, <code>push_front</code>, and <code>prepend_range</code> are already covered
by 23.2 <a href="https://wg21.link/container.requirements">[container.requirements]</a>. ]
</p>
</blockquote>

<pre>
iterator insert_after(const_iterator position, const T&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code>T</code> is <i>Cpp17CopyInsertable</i> into <code>forward_list</code>.
<code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).</ins>
</p>
<p>
<ins>-?- <i>Effects:</i> Inserts a copy of <code>x</code> after <code>position</code>.</ins>
</p>
<p>
<ins>-?- <i>Returns:</i> An iterator pointing to the copy of <code>x</code>.</ins>
</p>
</blockquote>

<pre>
iterator insert_after(const_iterator position, T&amp;&amp; x);
</pre>
<blockquote>
<p>
-6- <i>Preconditions:</i> <ins><code>T</code> is <i>Cpp17MoveInsertable</i> into <code>forward_list</code>.</ins>
    <code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).
</p>
<p>
-7- <i>Effects:</i> Inserts a copy of <code>x</code> after <code>position</code>.
</p>
<p>
-8- <i>Returns:</i> An iterator pointing to the copy of <code>x</code>.
</p>
</blockquote>

<pre>
iterator insert_after(const_iterator position, size_type n, const T&amp; x);
</pre>
<blockquote>
<p>
-9- <i>Preconditions:</i> <ins><code>T</code> is <i>Cpp17CopyInsertable</i> into <code>forward_list</code>.</ins>
    <code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).
</p>
<p>
-10- <i>Effects:</i> Inserts <code>n</code> copies of <code>x</code> after <code>position</code>.
</p>
<p>
-11- <i>Returns:</i> An iterator pointing to the last inserted copy of <code>x</code><ins>,</ins> or <code>position</code> 
     if <code>n == 0</code> <ins>is <code>true</code></ins>.
</p>
</blockquote>

<pre>
template&lt;class InputIterator&gt;	
  iterator insert_after(const_iterator position, InputIterator first, InputIterator last);
</pre>
<blockquote>
<p>
-12- <i>Preconditions:</i> <ins><code>T</code> is <i>Cpp17EmplaceConstructible</i> into <code>forward_list</code> from <code>*first</code>.</ins>
     <code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).
     Neither <code>first</code> nor <code>last</code> are iterators in <code>*this</code>.
</p>
<p>
-13- <i>Effects:</i> Inserts copies of elements in [<code>first</code>, <code>last</code>) after <code>position</code>.
</p>
<p>
-14- <i>Returns:</i> An iterator pointing to the last inserted element<ins>,</ins> or <code>position</code> if <code>first == last</code> 
     <ins>is <code>true</code></ins>.
</p>
</blockquote>

<pre>
template&lt;<i>container-compatible-range</i>&lt;T&gt; R&gt;
  iterator insert_after(const_iterator position, R&amp;&amp; rg);
</pre>
<blockquote>
<p>
-15- <i>Preconditions:</i> <ins><code>T</code> is <i>Cpp17EmplaceConstructible</i> into <code>forward_list</code> from <code>*ranges::begin(rg)</code>.</ins>
     <code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).
     <code>rg</code> and <code>*this</code> do not overlap.
</p>
<p>
-16- <i>Effects:</i> Inserts copies of elements in range <code>rg</code> after <code>position</code>.
</p>
<p>
-17- <i>Returns:</i> An iterator pointing to the last inserted element, or <code>position</code> if <code>rg</code> is empty.
</p>
</blockquote>

<pre>
iterator insert_after(const_iterator position, initializer_list&lt;T&gt; il);
</pre>
<blockquote>
<p>		
-18- <i>Effects:</i> <ins>Equivalent to: <code>return</code></ins> <code>insert_after(position, il.begin(), il.end())</code><del>.</del><ins>;</ins>
</p>
<p>
<del>-19- <i>Returns:</i> An iterator pointing to the last inserted element or <code>position</code> if <code>il</code> is empty.</del>
</p>
</blockquote>

<pre>
template&lt;class... Args&gt;	
  iterator emplace_after(const_iterator position, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-20- <i>Preconditions:</i> <ins><code>T</code> is <i>Cpp17EmplaceConstructible</i> into <code>forward_list</code> from 
     <code>std::forward&lt;Args&gt;(args)...</code>.</ins>
     <code>position</code> is <code>before_begin()</code> or is a dereferenceable iterator in the range [<code>begin()</code>, <code>end()</code>).
</p>
<p>
-21- <i>Effects:</i> Inserts an object of type <code>value_type</code> <del>constructed</del><ins>direct-non-list-initialized</ins> 
with <code><del>value_type(</del>std::forward&lt;Args&gt;(args)...<del>)</del></code> after <code>position</code>.
</p>
<p>
-22- <i>Returns:</i> An iterator pointing to the new object.
</p>
</blockquote>

</li>
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