lwg3545.html

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<title>std::pointer_traits should be SFINAE-friendly</title>
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<meta name="author" content="Glen Joseph Fernandes">
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<p><strong>Issue Number:</strong> 3545<br>
<strong>Date:</strong> 2021-04-26<br>
<strong>Project:</strong> Programming Language C++<br>
<strong>Audience:</strong> Library Working Group<br>
<strong>Author:</strong> Glen Joseph Fernandes
(<a href="mailto:glenjofe@gmail.com">glenjofe@gmail.com</a>)</p>
<h1>std::pointer_traits should be SFINAE-friendly</h1>
<h2>Discussion</h2>
<p>P1474R1 chose to use <code>std::to_address</code> (a mechanism of converting
pointer-like types to raw pointers) for contiguous iterators.
<code>std::to_address</code> provides an optional customization point via an
optional member in <code>std::pointer_traits</code>. However all iterators are
not pointers, and the primary template of
<code>std::pointer_traits&lt;Ptr&gt;</code> requires that either
<code>Ptr::element_type</code> is valid or <code>Ptr</code> is of the form
<code>template&lt;T, Args...&gt;</code> or the <code>pointer_traits</code>
specialization is ill-formed. This requires specializing
<code>pointer_traits</code> for those contiguous iterator types which is
inconvenient for users. P1474 should have also made <code>pointer_traits</code>
SFINAE friendly..</p>
<h2>Proposed resolution</h2>
<p>This resolution is relative to N4885.</p>
<p>1. Modify 20.10.3.2 [pointer.traits.types]/1 as indicated:</p>
<blockquote>
<p>-<em>Type</em>: <code>Ptr::element_type</code> if the <em>qualified-id</em>
<code>Ptr::element_type</code> is valid and denotes a type (13.10.3
[temp.deduct]); otherwise, <code>T</code> if <code>Ptr</code> is a class
template instantiation of the form <code>SomePointer&lt;T, Args&gt;</code>,
where <code>Args</code> is zero or more type arguments; otherwise, the
<del>specialization is ill-formed</del> <ins><code>pointer_traits</code> has no
member <code>element_type</code></ins>.</p>
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