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<title>Issue 4268: function&lt;void()&gt; suppresses nodiscard warnings</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#New">New</a> status.</em></p>
<h3 id="4268"><a href="lwg-active.html#4268">4268</a>. <code>function&lt;void()&gt;</code> suppresses <code class='backtick'>nodiscard</code> warnings</h3>
<p><b>Section:</b> 22.10.4 <a href="https://wg21.link/func.require">[func.require]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-05-29 <b>Last modified:</b> 2025-05-29</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#func.require">active issues</a> in [func.require].</p>
<p><b>View all other</b> <a href="lwg-index.html#func.require">issues</a> in [func.require].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<pre><code>struct [[nodiscard]] A { };
A f();
std::function&lt;void()&gt; func = f;
</code></pre>
<p>
Invoking <code class='backtick'>func()</code> will discard the return value of <code class='backtick'>f()</code>, but there will be
no warning. This is because <code><i>INVOKE</i>&lt;void&gt;(...)</code>
is defined in terms of <code>static_cast&lt;void&gt;(...)</code> and the
explicit cast to void suppresses <code class='backtick'>nodiscard</code> warnings.
This is in contast to <code><i>INVOKE</i>&lt;R&gt;(...)</code> where the
conversion to non-void <code class='backtick'>R</code> is implicit.
</p>

<p>
It seems right that <code>std::invoke_r&lt;void&gt;(f)</code> should not give
<code class='backtick'>nodiscard</code> warnings, because that's quite explicit about converting to void,
and similarly for <code>std::bind&lt;void&gt;(f)()</code>.
However, I think it's debatable whether all uses of <code><i>INVOKE</i>&lt;void&gt;</code> (and <code>std::function&lt;void()&gt;</code> in particular)
intend an explicit cast to void that ignores <code class='backtick'>nodiscard</code> types.
It's very easy to set <code class='backtick'>f</code> as the target of <code class='backtick'>func</code> and then lose its warning,
and there's no explicit use of <code class='backtick'>void</code> when you write <code class='backtick'>func = f; func();</code>.
</p>
<p>
We could consider defining <code><i>INVOKE</i>&lt;void&gt;(...)</code> to be
an expression of type void, without explicitly saying there's a cast to void.
For example, <code class='backtick'>(INVOKE(...), void())</code> would invoke the invocable and have type
<code class='backtick'>void</code>, but would not require any <code class='backtick'>nodiscard</code> warnings to be suppressed.
If we did that, some uses of <code><i>INVOKE</i>&lt;R&gt;</code> such as
<code>std::invoke_r</code> and <code>std::bind&lt;R&gt;</code> might need to
be adjusted to preserve the explicit conversion to void.
That would allow us to be selective about which uses of
<code><i>INVOKE</i>&lt;void&gt;</code> we consider to be explicit about
discarding results, and which we don't.
</p>


<p id="res-4268"><b>Proposed resolution:</b></p>
<p>
</p>





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