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<title>Issue 3264: sized_range and ranges::size redundantly use disable_sized_range</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="3264"><a href="lwg-defects.html#3264">3264</a>. <code>sized_range</code> and <code>ranges::size</code> redundantly use <code>disable_sized_range</code></h3>
<p><b>Section:</b> 25.4.4 <a href="https://wg21.link/range.sized">[range.sized]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2019-08-26 <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.sized">issues</a> in [range.sized].</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>
<code>disable_sized_range</code> (25.4.4 <a href="https://wg21.link/range.sized">[range.sized]</a>) is an opt-out trait that users may
specialize when their <code>range</code> type conforms to the syntax of <code>sized_range</code> but not its
semantics, or when the type is so poorly suited to the Standard Library that even testing the
validity of the expressions <code>r.size()</code> or <code>size(r)</code> for a range <code>r</code> is
impossible. The library inspects <code>disable_sized_range</code> in two places. (1) In the definition of
the <code>sized_range</code> concept:
<blockquote><pre>
template&lt;class T&gt;
  concept sized_range =
    range&lt;T&gt; &amp;&amp;
    !disable_sized_range&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp;
    requires(T&amp; t) { ranges::size(t); };
</pre></blockquote>
If the pertinent specialization of <code>disable_sized_range</code> is <code>true</code>, we avoid checking
the validity of the expression <code>ranges::size(t)</code> in the <i>requires-expression</i>.
(2) In the definition of the <code>ranges::size</code> CPO itself (25.3.10 <a href="https://wg21.link/range.prim.size">[range.prim.size]</a>),
the validity of the expressions <code><i>decay-copy</i>(E.size())</code> and
<code><i>decay-copy</i>(size(E))</code> is not checked if the pertinent specialization of
<code>disable_sized_range</code> is <code>true</code>.
</p><p>
<code>disable_sized_range</code> is effectively checked <em>twice</em> when evaluating
<code>sized_range</code>. This redundancy could be forgiven, if it did not permit the existence of
non-<code>sized_range</code>s for which <code>ranges::size</code> returns a valid size:
<blockquote><pre>
struct mytype {};
using A = mytype[42];

template &lt;&gt;
constexpr bool std::ranges::disable_sized_range&lt;A&gt; = true;

static_assert(std::ranges::range&lt;A&gt;);
static_assert(!std::ranges::sized_range&lt;A&gt;);
static_assert(std::ranges::size(A{}) == 42);

struct myrange {
    constexpr int* begin() const { return nullptr; }
    constexpr int* end() const { return nullptr; }
};

template &lt;&gt;
constexpr bool std::ranges::disable_sized_range&lt;myrange&gt; = true;

static_assert(std::ranges::range&lt;myrange&gt;);
static_assert(!std::ranges::sized_range&lt;myrange&gt;);
static_assert(std::ranges::size(myrange{}) == 0);
</pre></blockquote>
We should remove this gap between <code>ranges::size</code> and <code>sized_range</code> by checking
<code>disable_sized_range</code> only in the definition of <code>ranges::size</code>, and continuing to rely
on the validity of <code>ranges::size</code> in the <code>sized_range</code> concept.
</p>

<p><i>[2019-09-14 Priority set to 1 based on reflector discussion]</i></p>

<p><i>[2019-11 Wednesday night issue processing in Belfast.]</i></p>

<p>Status to Ready</p>


<p id="res-3264"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.4.4 <a href="https://wg21.link/range.sized">[range.sized]</a> as follows:</p>

<blockquote><pre>
template&lt;class T&gt;
  concept sized_range =
    range&lt;T&gt; &amp;&amp;
    <del>!disable_sized_range&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp;</del>
    requires(T&amp; t) { ranges::size(t); };
</pre></blockquote>
</li>
</ol>





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