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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>
      P3579R1
      [<a href="https://wg21.link/P3579">Latest</a>]
      [<a href="https://wg21.link/P3579/status">Status</a>]
    </td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-01-15</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="#changelog" id="toc-changelog"><span class="toc-section-number">2</span> Changelog<span></span></a>
<ul>
<li><a href="#since-r0" id="toc-since-r0"><span class="toc-section-number">2.0.1</span> Since R0<span></span></a></li>
</ul></li>
<li><a href="#overview" id="toc-overview"><span class="toc-section-number">3</span> Overview<span></span></a></li>
<li><a href="#partial-ordering" id="toc-partial-ordering"><span class="toc-section-number">4</span> Partial
ordering<span></span></a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">5</span> Wording<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">6</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="changelog"><span class="header-section-number">2</span> Changelog<a href="#changelog" class="self-link"></a></h1>
<h3 data-number="2.0.1" id="since-r0"><span class="header-section-number">2.0.1</span> Since R0<a href="#since-r0" class="self-link"></a></h3>
<ul>
<li>Added new section on partial ordering.</li>
<li>Wording improvements.</li>
</ul>
<h1 data-number="3" id="overview"><span class="header-section-number">3</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="4" id="partial-ordering"><span class="header-section-number">4</span> Partial ordering<a href="#partial-ordering" class="self-link"></a></h1>
<p>This issue also manifests itself in partial ordering:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode c++"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" 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> TT1<span class="op">&gt;</span> <span class="dt">void</span> f<span class="op">(</span>TT1<span class="op">&lt;</span><span class="dv">0</span><span class="op">&gt;);</span> <span class="co">// #1</span></span>
<span id="cb2-2"><a href="#cb2-2" 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">int</span><span class="op">&gt;</span> <span class="kw">class</span> TT2<span class="op">&gt;</span> <span class="dt">void</span> f<span class="op">(</span>TT2<span class="op">&lt;</span><span class="dv">0</span><span class="op">&gt;);</span>   <span class="co">// #2</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-4"><a href="#cb2-4" 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> B<span class="op">;</span></span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span> <span class="dt">void</span> f<span class="op">&lt;</span>B<span class="op">&gt;(</span>B<span class="op">&lt;</span><span class="dv">0</span><span class="op">&gt;);</span> <span class="co">// selects #2</span></span></code></pre></div>
<p>Before P0522, this selected
<code class="sourceCode cpp"><span class="pp">#2</span></code>.</p>
<p>After P0522, B matches both overloads, but they match each other
during partial ordering, so this is now ambiguous.</p>
<p>The change proposed in this paper will make it so
<code class="sourceCode cpp"><span class="pp">#1</span></code> doesn’t
match <code class="sourceCode cpp"><span class="pp">#2</span></code>,
making <code class="sourceCode cpp"><span class="pp">#2</span></code>
more specialized, restoring the original semantics.</p>
<h1 data-number="5" id="wording"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change <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 parameter PA from the
<em>template-head</em> of A, if a narrowing conversion ([dcl.init.list])
is required for initializing a variable of the same type as the
corresponding parameter of the <em>template-head</em> of P from a
variable of the same type as PA, the rewrite is ill-formed.</li>
</ul>

</div>
<p>If the rewrite <span class="rm" style="color: #bf0303"><del>produces
an invalid type</del></span><span class="add" style="color: #006e28"><ins>is ill-formed</ins></span>, then P is not at
least as specialized as A.</p>
<div class="add" style="color: #006e28">
<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 P&gt; struct S {};</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>template&lt;int&gt; struct A;</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>template struct S&lt;A&gt;; // OK, not narrowing</span></code></pre></div>
<p>Rewrite of the matching of the above template template parameter, in
terms of function template partial ordering:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode default c++"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>template&lt;int&gt; struct X {};        // Invented class X with template-head of A</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>template&lt;short PP&gt; void f(X&lt;PP&gt;); // #P</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>template&lt;int PP&gt; void f(X&lt;PP&gt;);   // #A</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>template void f&lt;0&gt;(X&lt;0&gt;);         // Invoke partial ordering for exposition only. OK: selects #P</span></code></pre></div>
<p>– end example]</p>
<p>[Example:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default c++"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>template&lt;template&lt;short&gt; class P&gt; struct S {};</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>template&lt;char&gt; struct B;</span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>template struct S&lt;B&gt;; // error: narrowing</span></code></pre></div>
<p>Rewrite of the matching of the above template template parameter, in
terms of function template partial ordering:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode default c++"><code class="sourceCode default"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>template&lt;char&gt; struct X {};       // Invented class X with template-head of B</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>template&lt;short PP&gt; void f(X&lt;PP&gt;); // #P</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a>template&lt;char PP&gt; void f(X&lt;PP&gt;);  // #A</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a>template void f&lt;0&gt;(X&lt;0&gt;);         // Invoke partial ordering for exposition only. Bad: narrowing conversion on #P</span></code></pre></div>
<p>As ordinary code, <code class="sourceCode default">#P</code> is
valid, and partial ordering would select
<code class="sourceCode default">#P</code>, but in the context of the
rewrite, the narrowing of PP to char is ill-formed.</p>
<p>– end example]</p>
<p>[Example:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode default c++"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>template&lt;template&lt;short&gt; class&gt; void f(); // #1</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>template&lt;template&lt;int&gt; class&gt; void f();   // #2</span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>template&lt;int&gt; struct B;</span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>template void f&lt;B&gt;(); // selects #2</span></code></pre></div>
<p>– end example]</p>
</div>
<h1 data-number="6" id="bibliography"><span class="header-section-number">6</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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