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    CWG Issue 1457</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="1457"></A><H4>1457.
  
Undefined behavior in left-shift
</H4>
<B>Section: </B>7.6.7&#160; [<A href="https://wg21.link/expr.shift">expr.shift</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD3
 &#160;&#160;&#160;

 <B>Submitter: </B>Howard Hinnant
 &#160;&#160;&#160;

 <B>Date: </B>2012-02-04<BR>


<P>[Moved to DR at the October, 2012 meeting.]</P>

<P>The current wording of 7.6.7 [<A href="https://wg21.link/expr.shift#2">expr.shift</A>] paragraph 2 makes it
undefined behavior to create the most-negative integer of a given type
by left-shifting a (signed) <TT>1</TT> into the sign bit, even though
this is not uncommonly done and works correctly on the majority of
(twos-complement) architectures:</P>

<BLOCKQUOTE>

...if <TT>E1</TT> has a signed type and non-negative value, and <TT>E1
&#10799; 2<SUP>E2</SUP></TT> is representable in the result type, then
that is the resulting value; otherwise, the behavior is undefined.

</BLOCKQUOTE>

<P>As a result, this technique cannot be used in a constant expression,
which will break a significant amount of code.</P>

<P><B>Proposed resolution (February, 2012):</B></P>

<P>Change 7.6.7 [<A href="https://wg21.link/expr.shift#2">expr.shift</A>] paragraph 2 as follows:</P>

<BLOCKQUOTE>

...if <TT>E1</TT> has a signed type and non-negative value, and <TT>E1
&#10799; 2<SUP>E2</SUP></TT> is representable in the <INS>corresponding
unsigned type of the</INS> result type, then that <INS>value,
converted to the result type,</INS> is the resulting value; otherwise,
the behavior is undefined.

</BLOCKQUOTE>

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