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<title>Issue 2413: assert macro is overconstrained</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#NAD">NAD</a> status.</em></p>
<h3 id="2413"><a href="lwg-closed.html#2413">2413</a>. <code>assert</code> macro is overconstrained</h3>
<p><b>Section:</b> 19.3 <a href="https://wg21.link/assertions">[assertions]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> David Krauss <b>Opened:</b> 2014-06-25 <b>Last modified:</b> 2023-06-13</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#assertions">issues</a> in [assertions].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When <code>NDEBUG</code> is defined, <code>assert</code> must expand exactly to the token sequence <code>((void)0)</code>, with no 
whitespace (C99 &sect;7.2/1 and also C11 &sect;7.2/1). This is a lost opportunity to pass the condition along to the optimizer.
<p/>
The user may observe the token sequence using the stringize operator or discriminate it by making a matching <code>#define</code> 
directive. There is little chance of practical code doing such things. It's reasonable to allow any expansion that is a <code>void</code> 
expression with no side effects or semantic requirements, for example, an extension keyword or an attribute-specifier finagled 
into the context.
<p/>
Conforming optimizations would still be limited to treating the condition as hint, not a requirement. Nonconformance on this 
point is quite reasonable though, given user preferences. Anyway, it shouldn't depend on preprocessor quirks.
<p/>
As for current practice, Darwin OS <code>&lt;assert.h&gt;</code> provides a GCC-style compiler hint <code>__builtin_expect</code> but only in 
debug mode. Shouldn't release mode preserve hints?
<p/>
Daniel:
<p/>
The corresponding resolution should take care not to conflict with the intention behind LWG <a href="lwg-defects.html#2234" title="assert() should allow usage in constant expressions (Status: C++17)">2234</a><sup><a href="https://cplusplus.github.io/LWG/issue2234" title="Latest snapshot">(i)</a></sup>.
</p>
<p><i>[2023-06-13, Varna; Status changed: New &rarr; NAD.]</i></p>

<p>
Only observable by stringifying an assert expression anyway,
so arguably allowed by implementations anyway.
Needs a paper to make changes here, or incorporate into <a href="https://wg21.link/P2884" title=" `assert` Should Be A Keyword In C++26">P2884</a>.
</p>


<p id="res-2413"><b>Proposed resolution:</b></p>





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