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    CWG Issue 1722</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="1722"></A><H4>1722.
  
Should lambda to function pointer conversion function be <TT>noexcept</TT>?
</H4>
<B>Section: </B>7.5.6.2&#160; [<A href="https://wg21.link/expr.prim.lambda.closure">expr.prim.lambda.closure</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD4
 &#160;&#160;&#160;

 <B>Submitter: </B>Ville Voutilainen
 &#160;&#160;&#160;

 <B>Date: </B>2013-07-31<BR>


<P>[Moved to DR at the October, 2015 meeting.]</P>



<P>According to 7.5.6 [<A href="https://wg21.link/expr.prim.lambda#6">expr.prim.lambda</A>] paragraph 6,</P>

<BLOCKQUOTE>

The closure type for a non-generic <I>lambda-expression</I> with
no <I>lambda-capture</I> has a public non-virtual non-explicit const
conversion function to pointer to function with C++ language linkage
(9.12 [<A href="https://wg21.link/dcl.link">dcl.link</A>]) having the same parameter and return types as
the closure type's function call operator.

</BLOCKQUOTE>

<P>This does not specify whether the conversion function is
<TT>noexcept(true)</TT> or <TT>noexcept(false)</TT>.  It might be
helpful to nail that down.</P>

<P><B>Proposed resolution (May, 2015):</B></P>

<P>Change 7.5.6 [<A href="https://wg21.link/expr.prim.lambda#6">expr.prim.lambda</A>] paragraph 6 as follows:</P>

<BLOCKQUOTE>

<P>The closure type for a non-generic <I>lambda-expression</I>
with no <I>lambda-capture</I> has a <DEL>public non-virtual
non-explicit const</DEL> conversion function to pointer to
function with C++ language linkage
(9.12 [<A href="https://wg21.link/dcl.link">dcl.link</A>]) having the same parameter and
return types as the closure type's function call
operator. The value returned by this conversion function
shall be the address of a function that, when invoked, has
the same effect as invoking the closure type's function call
operator. For a generic lambda with
no <I>lambda-capture</I>, the closure type has a <DEL>public
non-virtual non-explicit const</DEL> conversion function template
to pointer to function. The conversion function template...
[<I>Example:</I>
</P>

<PRE>
  auto GL = [](auto a) { std::cout &lt;&lt; a; return a; };
  int (*GL_int)(int) = GL; //<SPAN CLASS="cmnt"> OK: through conversion function template</SPAN>
  GL_int(3);               //<SPAN CLASS="cmnt"> OK: same as </SPAN>GL(3)
</PRE>

</BLOCKQUOTE>

<P>&#8212;<I>end example</I>] <INS>The conversion function or
conversion function template is public, non-virtual, non-explicit,
const, and has a non-throwing exception specification
(14.5 [<A href="https://wg21.link/except.spec">except.spec</A>]).</INS>
</P>

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