Merge 2023-11 CWG Motion 4

P2864R2 Remove Deprecated Arithmetic Conversion on Enumerations From C++26

Author: tkoeppe

source/compatibility.tex

@@ -13,6 +13,27 @@
 
 \rSec2[diff.cpp23.expr]{\ref{expr}: expressions}
 
+\diffref{expr.arith.conv}
+\change
+Operations mixing a value of an enumeration type and a value of a different
+enumeration type or of a floating-point type are no longer valid.
+\rationale
+Reinforcing type safety.
+\effect
+A valid \CppXXIII{} program that performs operations mixing a value of an
+enumeration type and a value of a different enumeration type or of a
+floating-point type is ill-formed.
+For example:
+\begin{codeblock}
+enum E1 { e };
+enum E2 { f };
+bool b = e <= 3.7;      // ill-formed; previously well-formed
+int  k = f - e;         // ill-formed; previously well-formed
+auto x = true ? e : f;  // ill-formed; previously well-formed
+\end{codeblock}
+
+\rSec2[diff.cpp23.dcl.dcl]{\ref{dcl.dcl}: Declarations}
+
 \diffref{dcl.init.list}
 \change
 Pointer comparisons between \tcode{initializer_list} objects' backing arrays
@@ -31,8 +52,6 @@
 bool b = ne({2,3}, {1,2,3});    // unspecified result; previously \tcode{false}
 \end{codeblock}
 
-\rSec2[diff.cpp23.dcl.dcl]{\ref{dcl.dcl}: Declarations}
-
 \diffref{dcl.array}
 \change
 Previously, \tcode{T...[n]} would declare a pack of function parameters.
@@ -2522,6 +2541,35 @@
 Some ISO C translators will give a warning
 if the cast is not used.
 
+\diffref{expr.arith.conv}
+\change
+Operations mixing a value of an enumeration type and a value of a different
+enumeration type or of a floating-point type are not valid.
+For example:
+\begin{codeblock}
+enum E1 { e };
+enum E2 { f };
+int b = e <= 3.7;       // valid in C; ill-formed in \Cpp{}
+int k = f - e;          // valid in C; ill-formed in \Cpp{}
+int x = 1 ? e : f;      // valid in C; ill-formed in \Cpp{}
+\end{codeblock}
+\rationale
+Reinforcing type safety in \Cpp{}.
+\effect
+Well-formed C code will not compile with this International Standard.
+\difficulty
+Violations will be diagnosed by the \Cpp{} translator.
+The original behavior can be restored with a cast or integral promotion.
+For example:
+\begin{codeblock}
+enum E1 { e };
+enum E2 { f };
+int b = (int)e <= 3.7;
+int k = +f - e;
+\end{codeblock}
+\howwide
+Uncommon.
+
 \diffref{expr.post.incr,expr.pre.incr}
 \change
 Decrement operator is not allowed with \keyword{bool} operand.

source/expressions.tex

@@ -1144,6 +1144,9 @@
 \item If either operand is of scoped enumeration type\iref{dcl.enum}, no conversions
 are performed; if the other operand does not have the same type, the expression is
 ill-formed.
+\item Otherwise, if one operand is of enumeration type and the other operand is
+of a different enumeration type or a floating-point type, the expression is
+ill-formed.
 \item Otherwise, if either operand is of floating-point type,
 the following rules are applied:
 \begin{itemize}
@@ -1197,13 +1200,6 @@
 \end{itemize}
 \end{itemize}
 
-\pnum
-If one operand is of enumeration type
-and the other operand is of
-a different enumeration type or
-a floating-point type,
-this behavior is deprecated\iref{depr.arith.conv.enum}.
-
 \rSec1[expr.prim]{Primary expressions}%
 \indextext{expression!primary|(}
 

source/future.tex

@@ -16,27 +16,6 @@
 An implementation may declare library names and entities described in this Clause with the
 \tcode{deprecated} attribute\iref{dcl.attr.deprecated}.
 
-\rSec1[depr.arith.conv.enum]{Arithmetic conversion on enumerations}
-
-\pnum
-The ability to apply the usual arithmetic conversions\iref{expr.arith.conv}
-on operands where one is of enumeration type
-and the other is of a different enumeration type
-or a floating-point type
-is deprecated.
-\begin{note}
-Three-way comparisons\iref{expr.spaceship} between such operands are ill-formed.
-\end{note}
-\begin{example}
-\begin{codeblock}
-enum E1 { e };
-enum E2 { f };
-bool b = e <= 3.7;              // deprecated
-int k = f - e;                  // deprecated
-auto cmp = e <=> f;             // error
-\end{codeblock}
-\end{example}
-
 \rSec1[depr.capture.this]{Implicit capture of \tcode{*this} by reference}
 
 \pnum

source/xrefdelta.tex

@@ -19,6 +19,9 @@
 %
 %   \removedxref{removed.label}
 
+% P2864R2 Remove deprecated arithmetic conversions
+\removedxref{depr.arith.conv.enum}
+
 % P2874R2 Mandating Annex D
 \removedxref{depr.res.on.required}