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    CWG Issue 2556</TITLE>
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<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="2556"></A><H4>2556.
  
Unusable <TT>promise::return_void</TT>
</H4>
<B>Section: </B>8.8.5&#160; [<A href="https://wg21.link/stmt.return.coroutine">stmt.return.coroutine</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Davis Herring
 &#160;&#160;&#160;

 <B>Date: </B>2022-03-24<BR>


<P>[Accepted as a DR at the November, 2023 meeting.]</P>



<P>Subclause 8.8.5 [<A href="https://wg21.link/stmt.return.coroutine#3">stmt.return.coroutine</A>] paragraph 3 specifies:</P>

<BLOCKQUOTE>

If <TT>p.return_void()</TT> is a valid expression, flowing off the end
of a coroutine's <I>function-body</I> is equivalent to
a <TT>co_return</TT> with no operand; otherwise flowing off the end of
a coroutine's <I>function-body</I> results in undefined behavior.

</BLOCKQUOTE>

<P>However, 9.6.4 [<A href="https://wg21.link/dcl.fct.def.coroutine#6">dcl.fct.def.coroutine</A>] paragraph 6 suggests:</P>

<BLOCKQUOTE>

If searches for the names <TT>return_void</TT>
and <TT>return_value</TT> in the scope of the promise type each find
any declarations, the program is ill-formed.  [<I>Note</I>: If
<TT>return_void</TT> is found, flowing off the end of a coroutine is
equivalent to a <TT>co_return</TT> with no operand.  Otherwise,
flowing off the end of a coroutine results in undefined behavior
(8.8.5 [<A href="https://wg21.link/stmt.return.coroutine">stmt.return.coroutine</A>]). &#8212;<I>end note</I>]

</BLOCKQUOTE>

<P>The difference is between the conditions "valid expression" and
"found by name lookup".  Effectively, it means that undefined behavior
might result where the implementation could instead diagnose an
ill-formed use of <TT>return_void</TT> (for example, because it is
inaccessible, deleted, or the function call requires arguments).</P>

<P><B>Proposed resolution (approved by CWG 2023-06-17):</B></P>

<P>Change in 8.8.5 [<A href="https://wg21.link/stmt.return.coroutine#3">stmt.return.coroutine</A>] paragraph 3 as follows:</P>

<BLOCKQUOTE>

If <DEL><TT>p.return_void()</TT> is a valid expression</DEL>
<INS>a search for the name <TT>return_void</TT> in the scope of the
promise type finds any declarations</INS>, flowing off the end of a
coroutine's <I>function-body</I> is equivalent to a <TT>co_return</TT>
with no operand; otherwise flowing off the end of a
coroutine's <I>function-body</I> results in undefined behavior.

</BLOCKQUOTE>

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