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  <title>Fix matching of non-type template parameters when matching
template template parameters</title>
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<h1 class="title" style="text-align:center">Fix matching of non-type
template parameters when matching template template parameters</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>
      D3579R0
      [<a href="https://wg21.link/P3579">Latest</a>]
      [<a href="https://wg21.link/P3579/status">Status</a>]
    </td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-01-12</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      Core Working Group<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Matheus Izvekov<br>&lt;<a href="mailto:mizvekov@gmail.com" class="email">mizvekov@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction" id="toc-introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#overview" id="toc-overview"><span class="toc-section-number">2</span> Overview<span></span></a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">3</span> Wording<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This paper is a follow-up to <span class="citation" data-cites="P3310R5"><a href="https://wg21.link/p3310r5" role="doc-biblioref">[P3310R5]</a></span>, and proposes to fix one
additional issue which came with the incorporation of <span class="citation" data-cites="P0522R0"><a href="https://wg21.link/p0522r0" role="doc-biblioref">[P0522R0]</a></span> into the standard.</p>
<p>The wording change failed to prevent a narrowing conversion for
non-type template parameters when matching a template
<em>template-argument</em> to a <em>template-parameter</em>.</p>
<p>This goes against the intent of P0522, which while it preserved the
type-theoretically incorrect parameter pack exception, aimed to not add
new such cases when matching template template parameters.</p>
<h1 data-number="2" id="overview"><span class="header-section-number">2</span> Overview<a href="#overview" class="self-link"></a></h1>
<p>Consider the following example:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode c++"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">template</span><span class="op">&lt;</span><span class="dt">short</span><span class="op">&gt;</span> <span class="kw">class</span><span class="op">&gt;</span> <span class="kw">struct</span> A <span class="op">{};</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="dt">short</span><span class="op">&gt;</span> <span class="kw">struct</span> B<span class="op">;</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="kw">struct</span> A<span class="op">&lt;</span>B<span class="op">&gt;;</span> <span class="co">// OK, exact match</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span> <span class="kw">struct</span> C<span class="op">;</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="kw">struct</span> A<span class="op">&lt;</span>C<span class="op">&gt;;</span> <span class="co">// #1: OK, all &#39;short&#39; values are valid &#39;int&#39; values</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="dt">char</span><span class="op">&gt;</span> <span class="kw">struct</span> D<span class="op">;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="kw">struct</span> A<span class="op">&lt;</span>D<span class="op">&gt;;</span> <span class="co">// #2: error, not all &#39;short&#39; values are valid &#39;char&#39; values</span></span></code></pre></div>
<p>The intention was that <span class="citation" data-cites="P0522R0"><a href="https://wg21.link/p0522r0" role="doc-biblioref">[P0522R0]</a></span> would allow
<code class="sourceCode cpp"><span class="pp">#1</span></code>, but it
inadvertently also allowed
<code class="sourceCode cpp"><span class="pp">#2</span></code>: the
wording change did not implement the paper’s intent in this case, which
assumed that the ‘at least as specialized’ check would only imply that
“any template argument list that can legitimately be applied to the
template template-parameter is also applicable to the argument
template”.</p>
<p>The new rules delegated this matching to partial ordering of function
templates, where a rewrite produced one each for the template parameter
and the template argument.</p>
<p>But when matching these function templates against each other in the
narrowing case, the template arguments produced from the non-type
template parameters are value-dependent, and narrowing conversions are
not diagnosed in this case, as in general dependent entities are not
diagnosed if they have valid instantiations.</p>
<h1 data-number="3" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Append to <span>13.4.4
<a href="https://wg21.link/temp.arg.template">[temp.arg.template]</a></span>/4</p>
<ul>
<li>Each function template has a single function parameter whose type is
a specialization of X with template arguments corresponding to the
template parameters from the respective function template where, for
each template parameter PP in the template-head of the function
template, a corresponding template argument AA is formed. If PP declares
a template parameter pack, then AA is the pack expansion PP…
([temp.variadic]); otherwise, AA is the id-expression PP.</li>
</ul>
<div class="add" style="color: #006e28">

<ul>
<li>For each non-type template argument AA, if a narrowing conversion
([dcl.init.list]) is required for initializing a variable of the same
type as PP from a variable of the same type as AA, the program is
ill-formed.</li>
</ul>
<p>[Example:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode default c++"><code class="sourceCode default"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>template&lt;template&lt;short&gt; class&gt; struct A {};</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>template&lt;int&gt; struct B;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>template struct A&lt;B&gt;; // OK, not narrowing</span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>template&lt;char&gt; struct C;</span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>template struct A&lt;C&gt;; // error: narrowing</span></code></pre></div>

</div>
<h1 data-number="4" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-P0522R0" class="csl-entry" role="doc-biblioentry">
[P0522R0] James Touton, Hubert Tong. 2016-11-11. DR: Matching of
template template-arguments excludes compatible templates. <a href="https://wg21.link/p0522r0"><div class="csl-block">https://wg21.link/p0522r0</div></a>
</div>
<div id="ref-P3310R5" class="csl-entry" role="doc-biblioentry">
[P3310R5] Matheus Izvekov. 2024-11-18. Solving partial ordering issues
introduced by relaxed template template parameter matching. <a href="https://wg21.link/p3310r5"><div class="csl-block">https://wg21.link/p3310r5</div></a>
</div>
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