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<HTML>
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<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
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CWG Issue 2911</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
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Core Issues List revision 114b.
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See http://www.open-std.org/jtc1/sc22/wg21/ for the official
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list.</EM></P>
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<P>2024-06-26</P>
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<HR>
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<A NAME="2911"></A><H4>2911.
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Unclear meaning of expressions "appearing within" subexpressions
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</H4>
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<B>Section: </B>7.5.7.1&#160; [<A href="https://wg21.link/expr.prim.req.general">expr.prim.req.general</A>]
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&#160;&#160;&#160;
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<B>Status: </B>open
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&#160;&#160;&#160;
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<B>Submitter: </B>Hubert Tong
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&#160;&#160;&#160;
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<B>Date: </B>2024-06-24<BR>
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<P>(From submission
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<A HREF="https://github.com/cplusplus/CWG/issues/562">#562</A>.)</P>
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<P>Subclause 7.5.7.1 [<A href="https://wg21.link/expr.prim.req.general#2">expr.prim.req.general</A>] paragraph 2 specifies:</P>
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<BLOCKQUOTE>
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A <I>requires-expression</I> is a prvalue of type bool whose value is
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described below. Expressions appearing within
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a <I>requirement-body</I> are unevaluated operands
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(7.2.3 [<A href="https://wg21.link/expr.context">expr.context</A>]).
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</BLOCKQUOTE>
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<P>A <I>constant-expression</I> used as a non-type template argument
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"appearing within" the <I>requirement-body</I> should not be
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considered an "unevaluated operand". Similarly, bodies
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of <I>lambda-expression</I>s should not be in focus of "appearing
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within".</P>
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<P><U>Suggested resolution:</U></P>
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<OL>
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<LI>
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<P>Change in 7.2.3 [<A href="https://wg21.link/expr.context#1">expr.context</A>] paragraph 1 as follows:</P>
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<BLOCKQUOTE>
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In some contexts, unevaluated operands appear
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(<DEL>7.5.7 [<A href="https://wg21.link/expr.prim.req">expr.prim.req</A>]</DEL> <INS>7.5.7.2 [<A href="https://wg21.link/expr.prim.req.simple">expr.prim.req.simple</A>],
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7.5.7.4 [<A href="https://wg21.link/expr.prim.req.compound">expr.prim.req.compound</A>]</INS>, 7.6.1.8 [<A href="https://wg21.link/expr.typeid">expr.typeid</A>],
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7.6.2.5 [<A href="https://wg21.link/expr.sizeof">expr.sizeof</A>], 7.6.2.7 [<A href="https://wg21.link/expr.unary.noexcept">expr.unary.noexcept</A>],
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9.2.9.6 [<A href="https://wg21.link/dcl.type.decltype">dcl.type.decltype</A>], 13.1 [<A href="https://wg21.link/temp.pre">temp.pre</A>],
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13.7.9 [<A href="https://wg21.link/temp.concept">temp.concept</A>]). An unevaluated operand is not evaluated.
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</BLOCKQUOTE>
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</LI>
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<LI>
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<P>Change in 7.5.7.1 [<A href="https://wg21.link/expr.prim.req.general#2">expr.prim.req.general</A>] paragraph 2 as follows:</P>
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<BLOCKQUOTE>
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A <I>requires-expression</I> is a prvalue of type bool whose value is
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described below. <DEL>Expressions appearing within
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a <I>requirement-body</I> are unevaluated operands
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(7.2.3 [<A href="https://wg21.link/expr.context">expr.context</A>]).</DEL>
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</BLOCKQUOTE>
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</LI>
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<LI>
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<P>Change in 7.5.7.2 [<A href="https://wg21.link/expr.prim.req.simple#1">expr.prim.req.simple</A>] paragraph 1 as follows:</P>
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<BLOCKQUOTE>
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A <I>simple-requirement</I> asserts the validity of an expression.
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<INS>The <I>expression</I> is an unevaluated operand.</INS>
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[<I>Note 1:</I> The enclosing <I>requires-expression</I> will evaluate
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to false if substitution of template arguments into the expression
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fails. <DEL>The expression is an unevaluated operand
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(7.2.3 [<A href="https://wg21.link/expr.context">expr.context</A>]).</DEL> &#8212;<I>end note</I>] ...
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</BLOCKQUOTE>
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</LI>
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<LI>
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<P>Change in 7.5.7.4 [<A href="https://wg21.link/expr.prim.req.compound#1">expr.prim.req.compound</A>] paragraph 1 as follows:</P>
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<BLOCKQUOTE>
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A <I>compound-requirement</I> asserts properties of the expression
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E. <INS>The <I>expression</I> is an unevaluated operand.</INS>
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Substitution of template arguments (if any) and verification of
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semantic properties proceed in the following order:
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</BLOCKQUOTE>
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</LI>
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</OL>
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<BR><BR>
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