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<title>Issue 3772: repeat_view's piecewise constructor is missing Postconditions</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="3772"><a href="lwg-defects.html#3772">3772</a>. <code>repeat_view</code>'s piecewise constructor is missing <i>Postconditions</i></h3>
<p><b>Section:</b> 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-12 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.repeat.view">issues</a> in [range.repeat.view].</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>
The first two value-bound pair constructors of <code>repeat_view</code> have the
<i>Preconditions</i> that the integer-like object <code>bound</code> must be
non-negative.
However, the piecewise constructor does not mention the valid values for
<code>bound_args</code>.
It would be nice to add a <i>Postconditions</i> that the initialized
<code>bound_</code> must be greater than or equal to <code>0</code> here.
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>
<p>
This is trying to state a requirement on users, but that's not what
<i>Postconditions:</i> means. Should be something more like:
<br/>
Precondition: If <code>Bound</code> is not <code>unreachable_sentinel_t</code>,
the  <code>bound_</code> &ge; 0 after its initialization from <code>bound_args</code>.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<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 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a> as shown:</p>

<blockquote>
<pre>
template&lt;class... WArgs, class... BoundArgs&gt;
  requires constructible_from&lt;W, WArgs...&gt; &amp;&amp;
           constructible_from&lt;Bound, BoundArgs...&gt;
constexpr explicit repeat_view(piecewise_construct_t,
  tuple&lt;Wargs...&gt; value_args, tuple&lt;BoundArgs...&gt; bound_args = tuple&lt;&gt;{});
</pre>

<blockquote>
<p>
-5- <i>Effects</i>: Initializes <code><i>value_</i></code> with arguments of types <code>WArgs...</code> obtained by forwarding the elements of <code>value_args</code> and initializes <code><i>bound_</i></code> 
with arguments of types <code>BoundArgs...</code> obtained by forwarding the elements of <code>bound_args</code>. (Here, forwarding an element <code>x</code> of type <code>U</code> within a tuple object means calling <code>std::forward&lt;U&gt;(x)</code>.)  
</p>
<p>
<ins>-?- <i>Postconditions</i>: If <code>Bound</code> is not <code>unreachable_sentinel_t</code>, <code><i>bound_</i></code> &ge; 0.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>

</blockquote>

<p><i>[2022-09-23; Jonathan provides improved wording]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<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 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a> as shown:</p>

<blockquote>
<pre>
template&lt;class... WArgs, class... BoundArgs&gt;
  requires constructible_from&lt;W, WArgs...&gt; &amp;&amp;
           constructible_from&lt;Bound, BoundArgs...&gt;
constexpr explicit repeat_view(piecewise_construct_t,
  tuple&lt;Wargs...&gt; value_args, tuple&lt;BoundArgs...&gt; bound_args = tuple&lt;&gt;{});
</pre>

<blockquote>
<p>
<ins>-?- <i>Preconditions</i>:
<code>Bound</code> is <code>unreachable_sentinel_t</code>,
or the initialization of <code><i>bound_</i></code> yields a non-negative value.
</ins>
</p>
<p>
-5- <i>Effects</i>: Initializes <code><i>value_</i></code> with arguments of types <code>WArgs...</code> obtained by forwarding the elements of <code>value_args</code> and initializes <code><i>bound_</i></code>
with arguments of types <code>BoundArgs...</code> obtained by forwarding the elements of <code>bound_args</code>. (Here, forwarding an element <code>x</code> of type <code>U</code> within a tuple object means calling <code>std::forward&lt;U&gt;(x)</code>.)
</p>
</blockquote>
</blockquote>
</li>
</ol>

</blockquote>

<p><i>[2023-01-11; Jonathan provides new wording requested by LWG]</i></p>


<p><i>[Issaquah 2023-02-07; LWG]</i></p>

<p>Move to Immediate for C++23</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3772"><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 25.6.5.2 <a href="https://wg21.link/range.repeat.view">[range.repeat.view]</a> as shown:</p>

<blockquote>
<pre>
template&lt;class... TArgs, class... BoundArgs&gt;
  requires constructible_from&lt;T, TArgs...&gt; &amp;&amp;
           constructible_from&lt;Bound, BoundArgs...&gt;
constexpr explicit repeat_view(piecewise_construct_t,
  tuple&lt;Targs...&gt; value_args, tuple&lt;BoundArgs...&gt; bound_args = tuple&lt;&gt;{});
</pre>

<blockquote>
<p>
-5- <i>Effects</i>: Initializes <code><i>value_</i></code> with
<del>arguments of types <code>TArgs...</code> obtained by
forwarding the elements of <code>value_args</code></del>
<ins><code>make_from_tuple&lt;T&gt;(std::move(value_args))</code></ins>
and initializes <code><i>bound_</i></code>
with
<del>arguments of types <code>BoundArgs...</code> obtained by forwarding the elements of <code>bound_args</code>. (Here, forwarding an element <code>x</code> of type <code>U</code> within a tuple object means calling <code>std::forward&lt;U&gt;(x)</code>.)
</del>
<ins><code>make_from_tuple&lt;Bound&gt;(std::move(bound_args))</code>.</ins>
<ins>
The behavior is undefined if
<code>Bound</code> is not <code>unreachable_sentinel_t</code>
and <code><i>bound_</i></code> is negative.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>






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