<HTML>
<HEAD>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<TITLE>
    CWG Issue 2745</TITLE>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<STYLE TYPE="text/css">
  INS { text-decoration:none; font-weight:bold; background-color:#A0FFA0 }
  .INS { text-decoration:none; background-color:#D0FFD0 }
  DEL { text-decoration:line-through; background-color:#FFA0A0 }
  .DEL { text-decoration:line-through; background-color: #FFD0D0 }
  @media (prefers-color-scheme: dark) {
    HTML { background-color:#202020; color:#f0f0f0; }
    A { color:#5bc0ff; }
    A:visited { color:#c6a8ff; }
    A:hover, a:focus { color:#afd7ff; }
    INS { background-color:#033a16; color:#aff5b4; }
    .INS { background-color: #033a16; }
    DEL { background-color:#67060c; color:#ffdcd7; }
    .DEL { background-color:#67060c; }
  }
  SPAN.cmnt { font-family:Times; font-style:italic }
</STYLE>
</HEAD>
<BODY>
<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="2745"></A><H4>2745.
  
Dependent odr-use in generic lambdas
</H4>
<B>Section: </B>6.3&#160; [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Shafik Yaghmour
 &#160;&#160;&#160;

 <B>Date: </B>2022-12-13<BR>


<P>[Accepted as a DR at the March, 2024 meeting.]</P>



<P>Default template arguments of generic lambdas can refer to local
variables.  It is unclear whether the potential odr-use is checked
when parsing the template definition or when instantiating the
template.</P>

<P>There is wide implementation divergence.</P>

<P><B>Proposed resolution (approved by CWG 2024-03-20):</B></P>

<P>Insert a new paragraph before 6.3 [<A href="https://wg21.link/basic.def.odr#11">basic.def.odr</A>] paragraph 11:</P>

<BLOCKQUOTE>

<P class="ins">
[ Example:
</P>
<PRE class="ins">
  void g() {
    constexpr int x = 1;
    auto lambda = [] &lt;typename T, int = ((T)x, 0)&gt; {};  //<SPAN CLASS="cmnt"> OK</SPAN>
    lambda.operator()&lt;int, 1&gt;();         //<SPAN CLASS="cmnt"> OK, does not consider x at all</SPAN>
    lambda.operator()&lt;int&gt;();            //<SPAN CLASS="cmnt"> OK, does not odr-use </SPAN>x
    lambda.operator()&lt;const int&amp;&gt;();     //<SPAN CLASS="cmnt"> error: odr-uses x from a context where x is not odr-usable</SPAN>
  }

  void h() {
    constexpr int x = 1;
    auto lambda = [] &lt;typename T&gt; { (T)x; };  //<SPAN CLASS="cmnt"> OK</SPAN>
    lambda.operator()&lt;int&gt;();            //<SPAN CLASS="cmnt"> OK, does not odr-use </SPAN>x
    lambda.operator()&lt;void&gt;();           //<SPAN CLASS="cmnt"> OK, does not odr-use </SPAN>x
    lambda.operator()&lt;const int&amp;&gt;();     //<SPAN CLASS="cmnt"> error: odr-uses x from a context where x is not odr-usable</SPAN>
  }
</PRE>
<P class="ins">
-- end example ]
</P>

<P>
Every program shall contain at least one definition of every function
or variable ...
</P>

</BLOCKQUOTE>

<BR><BR>
</BODY>
</HTML>
