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<title>Issue 3782: Should &lt;math.h&gt; declare ::lerp?</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#C++23">C++23</a> status.</em></p>
<h3 id="3782"><a href="lwg-defects.html#3782">3782</a>. Should <code>&lt;math.h&gt;</code> declare <code>::lerp</code>?</h3>
<p><b>Section:</b> 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-09-17 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#support.c.headers.other">active issues</a> in [support.c.headers.other].</p>
<p><b>View all other</b> <a href="lwg-index.html#support.c.headers.other">issues</a> in [support.c.headers.other].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
According to 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a>/1, <code>&lt;math.h&gt;</code> is required to 
provide <code>::lerp</code>, despite that it is a C++-only component. In <a href="https://wg21.link/P0811R3" title=" Well-behaved interpolation for numbers and pointers">P0811R3</a>, neither 
<code>&lt;math.h&gt;</code> nor C compatibility is mentioned, so it seems unintended not to list 
<code>lerp</code> as an exception in 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a>.
<p/>
Currently there is implementation divergence: libstdc++ provide <code>::lerp</code> in 
<code>&lt;math.h&gt;</code> but not in <code>&lt;cmath&gt;</code>, while MSVC STL and libc++ don't provide 
<code>::lerp</code> in <code>&lt;math.h&gt;</code> or <code>&lt;cmath&gt;</code> at all.
<p/>
I'm not sure whether this should be considered together with LWG <a href="lwg-active.html#3484" title="Should &lt;stddef.h&gt; declare ::nullptr_t? (Status: New)">3484</a><sup><a href="https://cplusplus.github.io/LWG/issue3484" title="Latest snapshot">(i)</a></sup>. 
Since <code>nullptr_t</code> has become a part of the C standard library as of C23 
(see <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm">WG14-N3042</a> and 
<a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3048.htm">WG14-N3048</a>), 
I believe we should keep providing <code>::nullptr_t</code> in <code>&lt;stddef.h&gt;</code>.
</p>

<p><i>[2022-10-12; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3782"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>

<li><p>Modify 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a> as indicated:</p>

<blockquote>
<p>
-1- Every C header other than <code>&lt;complex.h&gt;</code> (17.15.2 <a href="https://wg21.link/complex.h.syn">[complex.h.syn]</a>), 
<code>&lt;iso646.h&gt;</code> (17.15.3 <a href="https://wg21.link/iso646.h.syn">[iso646.h.syn]</a>), 
<code>&lt;stdalign.h&gt;</code> (17.15.4 <a href="https://wg21.link/stdalign.h.syn">[stdalign.h.syn]</a>),
<code>&lt;stdatomic.h&gt;</code> (32.5.12 <a href="https://wg21.link/stdatomic.h.syn">[stdatomic.h.syn]</a>), 
<code>&lt;stdbool.h&gt;</code> (17.15.5 <a href="https://wg21.link/stdbool.h.syn">[stdbool.h.syn]</a>), 
and <code>&lt;tgmath.h&gt;</code> (17.15.6 <a href="https://wg21.link/tgmath.h.syn">[tgmath.h.syn]</a>), each of which has a name of
the form <code>&lt;<i>name</i>.h&gt;</code>, behaves as if each name placed in 
the standard library namespace by the corresponding <code>&lt;c<i>name</i>&gt;</code> 
header is placed within the global namespace scope, except for the functions 
described in 29.7.6 <a href="https://wg21.link/sf.cmath">[sf.cmath]</a>, <ins>the <code>std::lerp</code> function overloads 
(29.7.4 <a href="https://wg21.link/c.math.lerp">[c.math.lerp]</a>),</ins> the declaration of <code>std::byte</code> 
(17.2.1 <a href="https://wg21.link/cstddef.syn">[cstddef.syn]</a>), and the functions and function templates described 
in 17.2.5 <a href="https://wg21.link/support.types.byteops">[support.types.byteops]</a>. [&hellip;]
</p>
</blockquote>
</li>

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