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    CWG Issue 651</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="651"></A><H4>651.
  
Problems in <TT>decltype</TT> specification and examples
</H4>
<B>Section: </B>9.2.9.3&#160; [<A href="https://wg21.link/dcl.type.simple">dcl.type.simple</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD1
 &#160;&#160;&#160;

 <B>Submitter: </B>Daveed Vandevoorde
 &#160;&#160;&#160;

 <B>Date: </B>16 Aug 2007<BR>


<P>[Voted into the WP at the September, 2008 meeting.]</P>

<P>The second bullet of 9.2.9.3 [<A href="https://wg21.link/dcl.type.simple#4">dcl.type.simple</A>] paragraph 4
reads,</P>

<BLOCKQUOTE>

<UL>
<LI>otherwise, if <TT>e</TT> is a function call (7.6.1.3 [<A href="https://wg21.link/expr.call">expr.call</A>]) or an invocation of an overloaded operator
(parentheses around <TT>e</TT> are ignored), <TT>decltype(e)</TT> is
the return type of that function;</LI>
</UL>

</BLOCKQUOTE>

<P>The reference to &#8220;that function&#8221; is imprecise; it is
not the actual function called at runtime but the statically chosen
function (ignoring covariant return types in virtual functions).</P>

<P>Also, the examples in this paragraph have errors:</P>

<OL>

<LI><P>The declaration of <TT>struct A</TT> should end with a
semicolon.</P></LI>

<LI><P>The lines of the form <TT>decltype(...);</TT> are ill-formed;
they need a declarator.</P></LI>

</OL>

<P><B>Proposed Resolution (October, 2007):</B></P>

<P>Change 9.2.9.3 [<A href="https://wg21.link/dcl.type.simple#4">dcl.type.simple</A>] paragraph 4 as follows:</P>

<BLOCKQUOTE>

<P>The type denoted by <TT>decltype(e)</TT> is defined as follows:</P>

<UL>

<LI><P>if <TT>e</TT> is an <I>id-expression</I> or a class member
access (7.6.1.5 [<A href="https://wg21.link/expr.ref">expr.ref</A>]), <TT>decltype(e)</TT> is the type
of the entity named by <TT>e</TT>. If there is no such entity, or
if <TT>e</TT> names a set of overloaded functions, the program is
ill-formed;</P></LI>

<LI><P>otherwise, if <TT>e</TT> is a function call (7.6.1.3 [<A href="https://wg21.link/expr.call">expr.call</A>]) or an invocation of an overloaded operator (parentheses
around <TT>e</TT> are ignored), <TT>decltype(e)</TT> is the return
type of <DEL>that</DEL> <INS>the statically chosen</INS> function;</P></LI>

<LI><P>otherwise, if <TT>e</TT> is an lvalue, <TT>decltype(e)</TT>
is <TT>T&amp;</TT>, where <TT>T</TT> is the type of <TT>e</TT>;</P></LI>

<LI><P>otherwise, <TT>decltype(e)</TT> is the type
of <TT>e</TT>.</P></LI>
</UL>

<P>The operand of the <TT>decltype</TT> specifier is an unevaluated
operand (Clause 7 [<A href="https://wg21.link/expr">expr</A>]).</P>

<P>[<I>Example:</I>
</P>
<PRE>
    const int&amp;&amp; foo();
    int i;
    struct A { double x; }<INS>;</INS>
    const A* a = new A();
    decltype(foo())<INS> x1</INS>;      //<SPAN CLASS="cmnt"> type is </SPAN>const int&amp;&amp;
    decltype(i)<INS> x2</INS>;          //<SPAN CLASS="cmnt"> type is </SPAN>int
    decltype(a-&gt;x)<INS> x3</INS>;       //<SPAN CLASS="cmnt"> type is </SPAN>double
    decltype((a-&gt;x))<INS> x4</INS>;     //<SPAN CLASS="cmnt"> type is </SPAN>const double&amp;
</PRE>
<P>&#8212;<I>end example</I>]</P>

</BLOCKQUOTE>

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