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<title>Issue 3282: subrange converting constructor should disallow derived to base conversions</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++20">C++20</a> status.</em></p>
<h3 id="3282"><a href="lwg-defects.html#3282">3282</a>. <code>subrange</code> converting constructor should disallow derived to base conversions</h3>
<p><b>Section:</b> 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2019-09-10 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.subrange">issues</a> in [range.subrange].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following code leads to slicing and general badness:
</p>
<blockquote><pre>
struct Base {};
struct Derived : Base {};
subrange&lt;Derived*&gt; sd;
subrange&lt;Base*&gt; sb = sd;
</pre></blockquote>
<p>
Traversal operations on iterators that are pointers do pointer arithmetic. If a <code>Base*</code> is 
actually pointing to a <code>Derived*</code>, then pointer arithmetic is invalid. <code>subrange</code>'s 
constructors can easily flag this invalid code, and probably should.
<p/>
The following PR incorporates the suggested fix to issue LWG <a href="lwg-defects.html#3281" title="Conversion from pair-like types to subrange is a silent semantic promotion (Status: C++20)">3281</a><sup><a href="https://cplusplus.github.io/LWG/issue3281" title="Latest snapshot">(i)</a></sup> I previously reported.
<p/>
Suggested priority: P1, since it will be hard to fix this after C++20 ships.
</p>
<p><i>[2019-10 Priority set to 1 and status to LEWG after reflector discussion]</i></p>

<p><i>[2019-10; Marshall comments]</i></p>

<p>
This issue would resolve <a href="https://github.com/cplusplus/nbballot/issues/281">US-285</a>.
</p>
<p><i>[2019-11 LEWG says OK; Status to Open. Friday PM discussion in Belfast. Casey to investigate and report back.]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  <ins>template&lt;class From, class To&gt;
    concept <i>convertible-to-non-slicing</i> = <i>// exposition only</i>
      convertible_to&lt;From, To&gt; &amp;&amp;
      !(is_pointer_v&lt;decay_t&lt;From&gt;&gt; &amp;&amp;
      is_pointer_v&lt;decay_t&lt;To&gt;&gt; &amp;&amp;
      <i>not-same-as</i>&lt;remove_pointer_t&lt;decay_t&lt;From&gt;&gt;, remove_pointer_t&lt;decay_t&lt;To&gt;&gt;&gt;);</ins>
      
  template&lt;class T&gt;
    concept <i>pair-like</i> = <i>// exposition only</i>
      [&hellip;]
      
  <del>template&lt;class T, class U, class V&gt;
    concept <i>pair-like-convertible-to</i> = <i>// exposition only</i>
      !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;remove_reference_t&lt;T&gt;&gt; &amp;&amp;
      requires(T&amp;&amp; t) {
        { get&lt;0&gt;(std::forward&lt;T&gt;(t)) } -&gt; convertible_to&lt;U&gt;;
        { get&lt;1&gt;(std::forward&lt;T&gt;(t)) } -&gt; convertible_to&lt;V&gt;;
      };</del>
      
   template&lt;class T, class U, class V&gt;
     concept <i>pair-like-convertible-from</i> = <i>// exposition only</i>
       !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;T&gt; &amp;&amp; 
       constructible_from&lt;T, U, V&gt; <ins>&amp;&amp;
       <i>convertible-to-non-slicing</i>&lt;U, tuple_element_t&lt;0, T&gt;&gt; &amp;&amp;
       convertible_to&lt;V, tuple_element_t&lt;1, T&gt;&gt;</ins>;

[&hellip;]
[&hellip;]
  template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S = I, subrange_kind K =
           sized_sentinel_for&lt;S, I&gt; ? subrange_kind::sized : subrange_kind::unsized&gt;
    requires (K == subrange_kind::sized || !sized_sentinel_for&lt;S, I&gt;)
  class subrange : public view_interface&lt;subrange&lt;I, S, K&gt;&gt; {
  private:
    [&hellip;]
  public:
    subrange() = default;
    
    constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s) requires (!StoreSize);
    
    constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s, 
                       <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n) requires (K == subrange_kind::sized);
    
    template&lt;<i>not-same-as</i>&lt;subrange&gt; R&gt;
      requires <i>forwarding-range</i>&lt;R&gt; &amp;&amp;
        <del>convertible_to</del><ins><i>convertible-to-non-slicing</i></ins>&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; 
        convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
    constexpr subrange(R&amp;&amp; r) requires (!StoreSize || sized_range&lt;R&gt;);
    
    template&lt;<i>forwarding-range</i> R&gt;
      requires convertible_to&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
    constexpr subrange(R&amp;&amp; r, <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n)
      requires (K == subrange_kind::sized)
        : subrange{ranges::begin(r), ranges::end(r), n}
    {}
    
    <del>template&lt;<i>not-same-as</i>&lt;subrange&gt; PairLike&gt;
      requires <i>pair-like-convertible-to</i>&lt;PairLike, I, S&gt;
    constexpr subrange(PairLike&amp;&amp; r) requires (!StoreSize)
      : subrange{std::get&lt;0&gt;(std::forward&lt;PairLike&gt;(r)),
                 std::get&lt;1&gt;(std::forward&lt;PairLike&gt;(r))}
    {}

    template&lt;<i>pair-like-convertible-to</i>&lt;I, S&gt; PairLike&gt;
    constexpr subrange(PairLike&amp;&amp; r, <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n)
      requires (K == subrange_kind::sized)
      : subrange{std::get&lt;0&gt;(std::forward&lt;PairLike&gt;(r)),
                 std::get&lt;1&gt;(std::forward&lt;PairLike&gt;(r)), n}
    {}</del>
  [&hellip;]
  };
  
  <ins>template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S&gt;
  subrange(I, S) -&gt; subrange&lt;I, S&gt;;</ins>  
  
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.5.4.2 <a href="https://wg21.link/range.subrange.ctor">[range.subrange.ctor]</a> as indicated:</p>

<blockquote>
<pre>
constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s) requires (!StoreSize);
</pre>
<blockquote>
<p>
-1- <i>Expects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s, 
                   <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n) requires (K == subrange_kind::sized);
</pre>
<blockquote>
<p>
-2- <i>Expects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;<i>not-same-as</i>&lt;subrange&gt; R&gt;
  requires <i>forwarding-range</i>&lt;R&gt; &amp;&amp;
    <del>convertible_to</del><ins><i>convertible-to-non-slicing</i></ins>&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; 
    convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
constexpr subrange(R&amp;&amp; r) requires (!StoreSize || sized_range&lt;R&gt;);
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2020-02-10; Prague]</i></p>

<p>
The group identified minor problems that have been fixed in the revised wording.
</p>

<p><i>[2020-02-10 Move to Immediate Monday afternoon in Prague]</i></p>



<p id="res-3282"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  <ins>template&lt;class From, class To&gt;
    concept <i>convertible-to-non-slicing</i> = <i>// exposition only</i>
      convertible_to&lt;From, To&gt; &amp;&amp;
      !(is_pointer_v&lt;decay_t&lt;From&gt;&gt; &amp;&amp;
      is_pointer_v&lt;decay_t&lt;To&gt;&gt; &amp;&amp;
      <i>not-same-as</i>&lt;remove_pointer_t&lt;decay_t&lt;From&gt;&gt;, remove_pointer_t&lt;decay_t&lt;To&gt;&gt;&gt;);</ins>
      
  template&lt;class T&gt;
    concept <i>pair-like</i> = <i>// exposition only</i>
      [&hellip;]
      
  <del>template&lt;class T, class U, class V&gt;
    concept <i>pair-like-convertible-to</i> = <i>// exposition only</i>
      !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;remove_reference_t&lt;T&gt;&gt; &amp;&amp;
      requires(T&amp;&amp; t) {
        { get&lt;0&gt;(std::forward&lt;T&gt;(t)) } -&gt; convertible_to&lt;U&gt;;
        { get&lt;1&gt;(std::forward&lt;T&gt;(t)) } -&gt; convertible_to&lt;V&gt;;
      };</del>
      
   template&lt;class T, class U, class V&gt;
     concept <i>pair-like-convertible-from</i> = <i>// exposition only</i>
       !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;T&gt; &amp;&amp; 
       constructible_from&lt;T, U, V&gt; <ins>&amp;&amp;
       <i>convertible-to-non-slicing</i>&lt;U, tuple_element_t&lt;0, T&gt;&gt; &amp;&amp;
       convertible_to&lt;V, tuple_element_t&lt;1, T&gt;&gt;</ins>;

[&hellip;]
[&hellip;]
  template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S = I, subrange_kind K =
           sized_sentinel_for&lt;S, I&gt; ? subrange_kind::sized : subrange_kind::unsized&gt;
    requires (K == subrange_kind::sized || !sized_sentinel_for&lt;S, I&gt;)
  class subrange : public view_interface&lt;subrange&lt;I, S, K&gt;&gt; {
  private:
    [&hellip;]
  public:
    subrange() = default;
    
    constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s) requires (!StoreSize);
    
    constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s, 
                       <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n) requires (K == subrange_kind::sized);
    
    template&lt;<i>not-same-as</i>&lt;subrange&gt; R&gt;
      requires <i>forwarding-range</i>&lt;R&gt; &amp;&amp;
        <del>convertible_to</del><ins><i>convertible-to-non-slicing</i></ins>&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; 
        convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
    constexpr subrange(R&amp;&amp; r) requires (!StoreSize || sized_range&lt;R&gt;);
    
    template&lt;<i>forwarding-range</i> R&gt;
      requires <del>convertible_to</del><ins><i>convertible-to-non-slicing</i></ins>&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; 
	    convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
    constexpr subrange(R&amp;&amp; r, <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n)
      requires (K == subrange_kind::sized)
        : subrange{ranges::begin(r), ranges::end(r), n}
    {}
    
    <del>template&lt;<i>not-same-as</i>&lt;subrange&gt; PairLike&gt;
      requires <i>pair-like-convertible-to</i>&lt;PairLike, I, S&gt;
    constexpr subrange(PairLike&amp;&amp; r) requires (!StoreSize)
      : subrange{std::get&lt;0&gt;(std::forward&lt;PairLike&gt;(r)),
                 std::get&lt;1&gt;(std::forward&lt;PairLike&gt;(r))}
    {}

    template&lt;<i>pair-like-convertible-to</i>&lt;I, S&gt; PairLike&gt;
    constexpr subrange(PairLike&amp;&amp; r, <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n)
      requires (K == subrange_kind::sized)
      : subrange{std::get&lt;0&gt;(std::forward&lt;PairLike&gt;(r)),
                 std::get&lt;1&gt;(std::forward&lt;PairLike&gt;(r)), n}
    {}</del>
  [&hellip;]
  };
  
  <ins>template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S&gt;
  subrange(I, S) -&gt; subrange&lt;I, S&gt;;</ins>  
  
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.5.4.2 <a href="https://wg21.link/range.subrange.ctor">[range.subrange.ctor]</a> as indicated:</p>

<blockquote>
<pre>
constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s) requires (!StoreSize);
</pre>
<blockquote>
<p>
-1- <i>Expects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
constexpr subrange(<ins><i>convertible-to-non-slicing</i>&lt;</ins>I<ins>&gt; auto</ins> i, S s, 
                   <i>make-unsigned-like-t</i>(iter_difference_t&lt;I&gt;) n) requires (K == subrange_kind::sized);
</pre>
<blockquote>
<p>
-2- <i>Expects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;<i>not-same-as</i>&lt;subrange&gt; R&gt;
  requires <i>forwarding-range</i>&lt;R&gt; &amp;&amp;
    <del>convertible_to</del><ins><i>convertible-to-non-slicing</i></ins>&lt;iterator_t&lt;R&gt;, I&gt; &amp;&amp; 
    convertible_to&lt;sentinel_t&lt;R&gt;, S&gt;
constexpr subrange(R&amp;&amp; r) requires (!StoreSize || sized_range&lt;R&gt;);
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> [&hellip;]
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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