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    CWG Issue 1740</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="1740"></A><H4>1740.
  
Disambiguation of <TT>noexcept</TT>
</H4>
<B>Section: </B>14.5&#160; [<A href="https://wg21.link/except.spec">except.spec</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++14
 &#160;&#160;&#160;

 <B>Submitter: </B>Richard Smith
 &#160;&#160;&#160;

 <B>Date: </B>2013-08-13<BR><BR>


<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3903.html#CA27">N3690 comment
  CA&#160;27<BR></A>

<P>[Moved to DR at the February, 2014 meeting.]</P>



<P>There is an ambiguity between a <TT>noexcept</TT> specifier's
optional parenthesized <I>constant-expression</I> and an initializer:</P>

<PRE>
  void f() noexcept;
  void (*p)() noexcept (&amp;f);
</PRE>

<P>Here, we can just about make 9.3.3 [<A href="https://wg21.link/dcl.ambig.res#1">dcl.ambig.res</A>] paragraph 1's rule
fit, and say that the <TT>(&amp;f)</TT> is part of the
<I>exception-specification</I> rather than being an initializer.  However,
this case is much more problematic:</P>

<PRE>
  void (*fp2)() noexcept, (*fp)() noexcept (fp2 = 0);
</PRE>

<P>The <TT>(fp = 0)</TT> here is unambiguously an initializer, because an
<I>assignment-expression</I> cannot syntactically be
a <I>constant-expression</I>, although current implementations treat it
as an ill-formed part of the <I>exception-specification</I>.</P>

<P>Probably the best approach would be to change 14.5 [<A href="https://wg21.link/except.spec">except.spec</A>]
to say that a <TT>(</TT> following <TT>noexcept</TT> is always treated
as being part of the <I>noexcept-specification</I>.</P>

<P><B>Proposed resolution (September, 2013):</B></P>

<P>Change 14.5 [<A href="https://wg21.link/except.spec#1">except.spec</A>] paragraph 1 as follows:</P>

<BLOCKQUOTE>

...In a <I>noexcept-specification</I>, the <I>constant-expression</I>, if
supplied, shall be a constant expression (7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]) that
is contextually converted to bool (7.3 [<A href="https://wg21.link/conv">conv</A>]). A
<I>noexcept-specification</I> <TT>noexcept</TT> is equivalent
to <TT>noexcept(<DEL> </DEL>true)</TT>. <INS>A <TT>(</TT> token that follows
<TT>noexcept</TT> is part of the <I>noexcept-specification</I> (and does
not commence an <I>initializer</I> (9.5 [<A href="https://wg21.link/dcl.init">dcl.init</A>]).</INS>

</BLOCKQUOTE>

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