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<title>Issue 4114: elements_view::iterator::operator* missing conditional noexcept specification</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="4114"><a href="lwg-active.html#4114">4114</a>. <code>elements_view::<i>iterator</i>::operator*</code> missing conditional <code>noexcept</code> specification</h3>
<p><b>Section:</b> 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-02 <b>Last modified:</b> 2024-08-02</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.elements.iterator">active issues</a> in [range.elements.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.elements.iterator">issues</a> in [range.elements.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>views::elements</code>, which can be seen as a specialization of <code>views::transform</code>, 
is used to <i>transform</i> the original tuple into its elements.
<p/>
Since neither has a specialization for <code>iter_move</code>, the customization point is dispatched 
to the default implementation. However, unlike the latter, <code>elements_view::<i>iterator</i>::operator*</code> 
does not have a <code>noexcept</code> specification, which makes calls to <code>iter_move</code> never 
<code>noexcept</code>, which seems to be an oversight (<a href="https://godbolt.org/z/aGe8EP6Wd">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;

std::vector v{std::pair{1, "a"}, {2, "b"}};

auto r1 = v | std::views::keys;
auto i1 = r1.begin();
static_assert(noexcept(*i1));                        // <span  style="color:#C80000;font-weight:bold">failed</span>
static_assert(noexcept(std::ranges::iter_move(i1))); // <span  style="color:#C80000;font-weight:bold">failed</span>

auto get_key = [](auto&amp; t) noexcept -&gt; auto&amp; { return std::get&lt;0&gt;(t); };
auto r2 = v | std::views::transform(get_key);
auto i2 = r2.begin();
static_assert(noexcept(*i2));
static_assert(noexcept(std::ranges::iter_move(i2)));
</pre></blockquote>
<p>
The proposed resolution is aligned with the <i>strengthened</i> implementation of MSVC-STL.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-4114"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>
<li><p>Modify 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a> as indicated:</p>

<blockquote><pre>
namespace std::ranges {
  template&lt;input_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; <i>has-tuple-element</i>&lt;range_value_t&lt;V&gt;, N&gt; &amp;&amp;
             <i>has-tuple-element</i>&lt;remove_reference_t&lt;range_reference_t&lt;V&gt;&gt;, N&gt; &amp;&amp;
             <i>returnable-element</i>&lt;range_reference_t&lt;V&gt;, N&gt;
  template&lt;bool Const&gt;
  class elements_view&lt;V, N&gt;::<i>iterator</i> {
    [&hellip;]

    static constexpr decltype(auto) <i>get-element</i>(const iterator_t&lt;<i>Base</i>&gt;&amp; i)<del>;</del>     // <i>exposition only</i>
      <ins>noexcept(noexcept(std::get&lt;N&gt;(*i))) requires is_reference_v&lt;range_reference_t&lt;<i>Base</i>&gt;&gt; {
      return std::get&lt;N&gt;(*i);
    }</ins>
    <ins>
    static constexpr decltype(auto) <i>get-element</i>(const iterator_t&lt;<i>Base</i>&gt;&amp; i)      // <i>exposition only</i>
      noexcept(noexcept(std::get&lt;N&gt;(*i)) &amp;&amp;
               is_nothrow_move_constructible_v&lt;tuple_element_t&lt;N, range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;) {
      using E = remove_cv_t&lt;tuple_element_t&lt;N, range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;;
      return static_cast&lt;E&gt;(std::get&lt;N&gt;(*i));
    }</ins>

  public:
    [&hellip;]

    constexpr decltype(auto) operator*() const
      <ins>noexcept(noexcept(<i>get-element</i>(<i>current_</i>)))</ins>
    { return <i>get-element</i>(<i>current_</i>); }

    [&hellip;]
  };
}
</pre></blockquote>
<p>
[&hellip;]
</p>
<pre><del>static constexpr decltype(auto) <i>get-element</i>(const iterator_t&lt;<i>Base</i>&gt;&amp; i);</del></pre>
<p>
<del>-3- <i>Effects</i>: Equivalent to:</del>
</p>
<blockquote><pre>
<del>if constexpr (is_reference_v&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;) {
  return std::get&lt;N&gt;(*i);
} else {
  using E = remove_cv_t&lt;tuple_element_t&lt;N, range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;;
  return static_cast&lt;E&gt;(std::get&lt;N&gt;(*i));
}</del>
</pre></blockquote>

</li>
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