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  <meta name="dcterms.date" content="2024-04-16" />
  <title>Allow `[[nodiscard]]` in type alias declarations</title>
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      code span { } /* Normal */
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      code span.an { } /* Annotation */
      code span.at { } /* Attribute */
      code span.bn { color: #9f6807; } /* BaseN */
      code span.bu { color: #9f6807; } /* BuiltIn */
      code span.cf { color: #00607c; } /* ControlFlow */
      code span.ch { color: #9f6807; } /* Char */
      code span.cn { } /* Constant */
      code span.co { color: #008000; font-style: italic; } /* Comment */
      code span.cv { color: #008000; font-style: italic; } /* CommentVar */
      code span.do { color: #008000; } /* Documentation */
      code span.dt { color: #00607c; } /* DataType */
      code span.dv { color: #9f6807; } /* DecVal */
      code span.er { color: #ff0000; font-weight: bold; } /* Error */
      code span.ex { } /* Extension */
      code span.fl { color: #9f6807; } /* Float */
      code span.fu { } /* Function */
      code span.im { } /* Import */
      code span.in { color: #008000; } /* Information */
      code span.kw { color: #00607c; } /* Keyword */
      code span.op { color: #af1915; } /* Operator */
      code span.ot { } /* Other */
      code span.pp { color: #6f4e37; } /* Preprocessor */
      code span.re { } /* RegionMarker */
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      code span.va { } /* Variable */
      code span.vs { color: #9f6807; } /* VerbatimString */
      code span.wa { color: #008000; font-weight: bold; } /* Warning */
      code.diff {color: #898887}
      code.diff span.va {color: #006e28}
      code.diff span.st {color: #bf0303}
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<h1 class="title" style="text-align:center">Allow <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
in type alias declarations</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3245R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-04-16</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>
      Evolution Working Group Incubator<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Xavier Bonaventura (<a href="https://www.bmwgroup.com/en/innovation/automated-driving.html">BMW</a>)<br><xavier.bonaventura@bmw.de><br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<h1 data-number="1" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>This paper proposes to allow the usage of <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
to be used in type alias declarations in the same way that can be done
with <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>.</p>
<h1 data-number="2" id="current-usage-of-nodiscard-in-types"><span class="header-section-number">2</span> Current usage of <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
in types<a href="#current-usage-of-nodiscard-in-types" class="self-link"></a></h1>
<p><code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
was initially proposed in <span class="citation" data-cites="P0068R0">[<a href="https://wg21.link/p0068r0" role="doc-biblioref">P0068R0</a>]</span> and <span class="citation" data-cites="P0189R1">[<a href="https://wg21.link/p0189r1" role="doc-biblioref">P0189R1</a>]</span> to be used in types and
functions. For types, it can be used in the following way:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> <span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span> Error<span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>    <span class="dt">int</span> code;</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>MyError critical_call<span class="op">(){</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>    <span class="cf">return</span> <span class="op">{</span><span class="dv">42</span><span class="op">}</span>;</span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</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="dt">int</span> main<span class="op">(){</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a>    <span class="co">// Most compilers will issue diagnostics on the next call</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a>    <span class="co">// for any function that returns the MyError type</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a>    critical_call<span class="op">()</span>;</span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>This allows users to notify the implementation that any function
returning a type <code class="sourceCode cpp">Error</code>, its return
value should not be discarded silently.</p>
<h1 data-number="3" id="the-missing-use-case"><span class="header-section-number">3</span> The missing use case<a href="#the-missing-use-case" class="self-link"></a></h1>
<p>There is one use case that is missing with the current allowed usage
of the <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
attribute that would be useful.</p>
<p>Imagine an external library that defines a type
<code class="sourceCode cpp">Error</code> that we want to use in our
library. Because this is conceptually an error type, in a safety
critical sytem you might be interested on having this type treated as
<code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>.</p>
<p>If the library already marks the type as <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
then everything is good an you can use it directly or using an alias. If
that is not the case, then it is not so simple.</p>
<p>Intuitively we would put the <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
attribute at the same place like it is allowed nowadays with <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">using</span> MyError <span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span> <span class="op">=</span> Error;</span></code></pre></div>
<p>However, the attribute above will be ignored. The grammer allows the
attribute to be in such a position but it is not allowed by the <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
attribute specification <span>9.12.9
<a href="https://wg21.link/dcl.attr.nodiscard">[dcl.attr.nodiscard]</a></span>.</p>
<h1 data-number="4" id="proposal"><span class="header-section-number">4</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>The proposal would be to allow the usage of <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
in type aliases. This is already possible in a none standard way using
compiler annotations.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Before</strong>
</div></th>
<th><div style="text-align:center">
<strong>After</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="co">// In clang</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="kw">using</span> MyError <span class="op">[[</span><span class="ex">clang::warn_unused_result</span><span class="op">]]</span> <span class="op">=</span> Error; </span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a><span class="co">// In gcc</span></span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a><span class="kw">using</span> MyError <span class="ex">__attribute__((warn_unused_result))</span> <span class="op">=</span> Error;</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a><span class="co">// In MSVC</span></span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="co">// I&#39;m not aware that is possible</span></span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="kw">using</span> MyError <span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span> <span class="op">=</span> Error;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<h2 data-number="4.1" id="problems-and-potential-solutions"><span class="header-section-number">4.1</span> Problems and potential
solutions<a href="#problems-and-potential-solutions" class="self-link"></a></h2>
<p>The main problem with the proposal above is that an alias declaration
is not a type. This is a big difference when trying to apply <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
in comparison to <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>.
In the case of <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>,
the compiler can issue a warning in the moment that it sees the name in
the declaration. For the case of <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>,
the compiler would have to keep this information additionally because
the alias is not a type.</p>
<p>In this section we will present the two possible solutions that could
be done. I would like to get feedback from the committee which one is
preferred.</p>
<h3 data-number="4.1.1" id="aliases-carry-nodiscard-information"><span class="header-section-number">4.1.1</span> Aliases carry <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
information<a href="#aliases-carry-nodiscard-information" class="self-link"></a></h3>
<p>This option would require to remember if a function was seen with the
alias or with the original type. This would mean that if the compiler
has seen the function with the return alias, it should issue the warning
when the return value is not used.</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>Error foo<span class="op">(</span><span class="dt">int</span> bar<span class="op">)</span>;</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>foo<span class="op">(</span><span class="dv">42</span><span class="op">)</span>; <span class="co">// No warning is issued`</span></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>MyError foo<span class="op">(</span><span class="dt">int</span> bar<span class="op">)</span>; <span class="co">// Same declaration like above due to Error and MyError being the same type</span></span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a>foo<span class="op">(</span><span class="dv">42</span><span class="op">)</span>; <span class="co">// `[[nodiscard]] warning is issued`</span></span></code></pre></div>
<h3 data-number="4.1.2" id="alias-mechanism-to-introduce-a-type-with-different-discardability-semantics"><span class="header-section-number">4.1.2</span> Alias mechanism to introduce
a type with different discardability semantics<a href="#alias-mechanism-to-introduce-a-type-with-different-discardability-semantics" class="self-link"></a></h3>
<p>Another direction could be considered is to introduce a mechanism to
introduce a type from another one. In this case, this would mean that
<code class="sourceCode cpp">Error</code> and
<code class="sourceCode cpp">MyError</code> would be two different
types, one that is <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
and one that is not.</p>
<h1 data-number="5" id="wording"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>There is no wording yet, first the direction needs to be discussed.
Depending on that the work on the wording will be done.</p>
<h1 data-number="6" id="acknowledgements"><span class="header-section-number">6</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Thanks a lot to everyone that participated in the initial discussion
in Mattermost and during the Tokyo meeting. This allowed me to create
the first version of the paper already exposing the difference between
<code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
and <code class="sourceCode cpp"><span class="op">[[</span><span class="at">deprecated</span><span class="op">]]</span></code>
and exposing the challange with adding <code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
in type aliases.</p>
<h1 data-number="7" id="bibliography"><span class="header-section-number">7</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-P0068R0" class="csl-entry" role="doc-biblioentry">
[P0068R0] Andrew Tomazos. 2015-09-03. Proposal of [[unused]],
[[nodiscard]] and [[fallthrough]] attributes. <a href="https://wg21.link/p0068r0"><div class="csl-block">https://wg21.link/p0068r0</div></a>
</div>
<div id="ref-P0189R1" class="csl-entry" role="doc-biblioentry">
[P0189R1] Andrew Tomazos. 2016-02-29. Wording for [[nodiscard]]
attribute. <a href="https://wg21.link/p0189r1"><div class="csl-block">https://wg21.link/p0189r1</div></a>
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