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    CWG Issue 2517</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="2517"></A><H4>2517.
  
Useless restriction on use of parameter in <I>constraint-expression</I>
</H4>
<B>Section: </B>7.5.8.5&#160; [<A href="https://wg21.link/expr.prim.req.nested">expr.prim.req.nested</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++23
 &#160;&#160;&#160;

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

 <B>Date: </B>2019-06-10<BR>


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



<P>According to 7.5.8.5 [<A href="https://wg21.link/expr.prim.req.nested#2">expr.prim.req.nested</A>] paragraph 2,</P>

<BLOCKQUOTE>

<P>A local parameter shall only appear as an unevaluated
operand (7.2.3 [<A href="https://wg21.link/expr.context">expr.context</A>]) within
the <I>constraint-expression</I>.  [<I>Example 2</I>:</P>

<PRE>
  template&lt;typename T&gt; concept C = requires (T a) {
    requires sizeof(a) == 4; //<SPAN CLASS="cmnt"> OK</SPAN>
    requires a == 0; //<SPAN CLASS="cmnt"> error: evaluation of a constraint variable</SPAN>
  };
</PRE>

<P>&#8212;<I>end example</I>]</P>

</BLOCKQUOTE>

<P>However, <TT>a</TT> can't be used in a constant
expression in any event, so the restriction is
meaningless, except for ruling out an expression like
<TT>true ? true : a</TT>, but there seems no reason to
have a special rule for such a case.</P>

<P><B>Proposed resolution (approved by CWG 2023-01-06):</B></P>

Remove 7.5.8.5 [<A href="https://wg21.link/expr.prim.req.nested#2">expr.prim.req.nested</A>] paragraph 2, including its example:

<BLOCKQUOTE>

<P><DEL>A local parameter shall only appear as an unevaluated
operand (7.2.3 [<A href="https://wg21.link/expr.context">expr.context</A>]) within
the <I>constraint-expression</I>.  [<I>Example 2</I>:</DEL></P>

<PRE>
  <DEL>template&lt;typename T&gt; concept C = requires (T a) {
    requires sizeof(a) == 4; //<SPAN CLASS="cmnt"> OK</SPAN>
    requires a == 0; //<SPAN CLASS="cmnt"> error: evaluation of a constraint variable</SPAN>
  };</DEL>
</PRE>

</BLOCKQUOTE>

<P><B>Additional notes (February, 2023)</B></P>

<P>After adopting paper P2280, it is no longer accurate that any use
of requirement parameters makes an expression non-constant.  However,
the resolution as adopted makes the treatment of examples like the
following uniform in requirements and other constant expression
contexts:</P>

<PRE>
  template&lt;typename ArrayType&gt; concept LargeArray =
    requires (ArrayType my_array) { requires my_array.size() &gt; 5; }
</PRE>

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