P1120R0 Consistency improvements for <=> and other comparison operators

Author: zygoloid

source/expressions.tex

@@ -474,6 +474,13 @@
 \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|(}
 
@@ -4964,6 +4971,8 @@
 
 \pnum
 If both operands have arithmetic types,
+or one operand has integral type and
+the other operand has unscoped enumeration type,
 the usual arithmetic conversions\iref{expr.arith.conv} are applied to the operands.
 Then:
 
@@ -5092,7 +5101,7 @@
 \tcode{a<b<c} means \tcode{(a<b)<c} and \emph{not}
 \tcode{(a<b)\&\&(b<c)}.
 \end{example}
-
+%
 \indextext{operator!less than}%
 \indextext{\idxcode{<}|see{operator, less than}}%
 \indextext{operator!greater than}%
@@ -5110,8 +5119,19 @@
     relational-expression \terminal{<=} compare-expression\br
     relational-expression \terminal{>=} compare-expression
 \end{bnf}
+%
+The
+lvalue-to-rvalue\iref{conv.lval},
+array-to-pointer\iref{conv.array},
+and function-to-pointer\iref{conv.func}
+standard conversions are performed on the operands.
+The comparison is deprecated if
+both operands were of array type
+prior to these conversions\iref{depr.array.comp}.
 
-The operands shall have arithmetic, enumeration, or pointer type. The
+\pnum
+The converted operands shall have arithmetic, enumeration, or pointer type.
+The
 operators \tcode{<} (less than), \tcode{>} (greater than), \tcode{<=}
 (less than or equal to), and \tcode{>=} (greater than or equal to) all
 yield \tcode{false} or \tcode{true}. The type of the result is
@@ -5178,7 +5198,18 @@
 
 \pnum
 The \tcode{==} (equal to) and the \tcode{!=} (not equal to) operators
-group left-to-right. The operands shall have arithmetic, enumeration, pointer,
+group left-to-right.
+The
+lvalue-to-rvalue\iref{conv.lval},
+array-to-pointer\iref{conv.array},
+and function-to-pointer\iref{conv.func}
+standard conversions are performed on the operands.
+The comparison is deprecated if
+both operands were of array type
+prior to these conversions\iref{depr.array.comp}.
+
+\pnum
+The converted operands shall have arithmetic, enumeration, pointer,
 or pointer-to-member type, or type \tcode{std::nullptr_t}. The operators
 \tcode{==} and \tcode{!=} both yield \tcode{true} or \tcode{false}, i.e., a
 result of type \tcode{bool}. In each case below, the operands shall have the

source/future.tex

@@ -14,6 +14,27 @@
 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;             // ill-formed
+\end{codeblock}
+\end{example}
+
 \rSec1[depr.capture.this]{Implicit capture of \tcode{*this} by reference}
 
 \pnum
@@ -33,6 +54,25 @@
 \end{codeblock}
 \end{example}
 
+\rSec1[depr.array.comp]{Array comparisons}
+
+\pnum
+Equality and relational comparisons (\ref{expr.eq}, \ref{expr.rel})
+between two operands of array type
+are deprecated.
+\begin{note}
+Three-way comparisons\iref{expr.spaceship} between such operands are ill-formed.
+\end{note}
+\begin{example}
+\begin{codeblock}
+int arr1[5];
+int arr2[5];
+bool same = arr1 == arr2;       // deprecated, same as \tcode{\&arr[0] == \&arr[1]},
+                                // does not compare array contents
+auto cmp = arr1 <=> arr2;       // ill-formed
+\end{codeblock}
+\end{example}
+
 \rSec1[depr.static_constexpr]{Redeclaration of \tcode{static constexpr} data members}
 
 \pnum