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    CWG Issue 2338</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="2338"></A><H4>2338.
  
Undefined behavior converting to short enums with fixed underlying types
</H4>
<B>Section: </B>7.6.1.9&#160; [<A href="https://wg21.link/expr.static.cast">expr.static.cast</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD5
 &#160;&#160;&#160;

 <B>Submitter: </B>Thomas K&#246;ppe
 &#160;&#160;&#160;

 <B>Date: </B>2017-03-24<BR>


<P>[Accepted as a DR at the November, 2017 meeting.]</P>

<P>The specifications of <TT>std::byte</TT>
(17.2.5 [<A href="https://wg21.link/support.types.byteops">support.types.byteops</A>]) and <TT>bitmask</TT>
(16.3.3.3.3 [<A href="https://wg21.link/bitmask.types">bitmask.types</A>]) have revealed a problem with
the integral conversion rules, according to which both those
specifications have, in the general case. undefined behavior. The
problem is that a conversion to an enumeration type has undefined
behavior unless the value to be converted is in the range of the
enumeration.</P>

<P>For enumerations with an unsigned fixed underlying type, this
requirement is overly restrictive, since converting a large value
to an unsigned integer type is well-defined.</P>

<P><B>Proposed resolution (August, 2017):</B></P>

<P>Change 7.6.1.9 [<A href="https://wg21.link/expr.static.cast#10">expr.static.cast</A>] paragraph 10 as follows:</P>

<BLOCKQUOTE>

A value of integral or enumeration type can be explicitly
converted to a complete enumeration type. <DEL>The</DEL> <INS>If
the enumeration type has a fixed underlying type, the value is
first converted to that type by integral conversion, if
necessary, and then to the enumeration type. If the enumeration
type does not have a fixed underlying type, the</INS> value is
unchanged if the original value is within the range of the
enumeration values
(9.8.1 [<A href="https://wg21.link/dcl.enum">dcl.enum</A>])<DEL>. Otherwise</DEL><INS>, and
otherwise</INS>, the behavior is undefined.

</BLOCKQUOTE>

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