P1630R1 Spaceship needs a tune-up

Definition of 'usable function' not added: it will be added by CWG
motion 6 (P1186R3) instead.

Author: zygoloid

source/classes.tex

@@ -6661,6 +6661,12 @@
 \item a friend of \tcode{C} having two parameters of type \tcode{const C\&}.
 \end{itemize}
 
+\pnum
+A defaulted comparison operator function for class \tcode{C}
+is defined as deleted if
+any non-static data member of \tcode{C} is of reference type or
+\tcode{C} is a union-like class\iref{class.union.anon}.
+
 \pnum
 If the class definition
 does not explicitly declare an \tcode{==} operator function,
@@ -6697,15 +6703,22 @@
 given a glvalue \tcode{x} of type \tcode{const C}, either:
 \begin{itemize}
 \item
-  \tcode{C} is a non-class type and \tcode{x <=> x} is a valid expression
-  of type \tcode{std::strong_ordering} or \tcode{std::strong_equality}, or
+\tcode{C} is a non-class type and
+\tcode{x <=> x} is a valid expression
+of type \tcode{std::strong_ordering} or \tcode{std::strong_equality}, or
+
 \item
-  \tcode{C} is a class type with an \tcode{==} operator
-  defined as defaulted in the definition of \tcode{C},
-  \tcode{x == x} is well-formed when contextually converted to \tcode{bool},
-  all of \tcode{C}'s base class subobjects and non-static data members
-  have strong structural equality, and
-  \tcode{C} has no \tcode{mutable} or \tcode{volatile} subobjects.
+\tcode{C} is a class type
+where all of the following hold:
+\begin{itemize}
+\item All of \tcode{C}'s base class subobjects and non-static data members
+have strong structural equality.
+\item \tcode{C} has no mutable or volatile non-static data members.
+\item At the end of the definition of \tcode{C},
+overload resolution performed for the expression \tcode{x == x} succeeds and
+finds either a friend or public member \tcode{==} operator
+that is defined as defaulted in the definition of \tcode{C}.
+\end{itemize}
 \end{itemize}
 
 \pnum
@@ -6760,15 +6773,12 @@
 with parameters \tcode{x} and \tcode{y} is defined as deleted if
 \begin{itemize}
 \item
-  overload resolution\iref{over.match}, as applied to \tcode{x == y}
-  (also considering synthesized candidates
-  with reversed order of parameters\iref{over.match.oper}),
-  results in an ambiguity or a function
-  that is deleted or inaccessible from the operator function, or
+  overload resolution\iref{over.match}, as applied to \tcode{x == y},
+  does not result in a usable function, or
 \item
-  \tcode{x == y} cannot be contextually converted to \tcode{bool}.
+  \tcode{x == y} is not a prvalue of type \tcode{bool}.
 \end{itemize}
-Otherwise, the operator function yields \tcode{(x == y) ?\ false :\ true}.
+Otherwise, the operator function yields \tcode{!(x == y)}.
 
 \pnum
 \begin{example}
@@ -6777,7 +6787,7 @@
   int i;
   friend bool operator==(const D& x, const D& y) = default;
                                                 // OK, returns \tcode{x.i == y.i}
-  bool operator!=(const D& z) const = default;  // OK, returns \tcode{(*this == z) ?\ false :\ true}
+  bool operator!=(const D& z) const = default;  // OK, returns \tcode{!(*this == z)}
 };
 \end{codeblock}
 \end{example}
@@ -6874,10 +6884,8 @@
 \begin{itemize}
 \item
 overload resolution\iref{over.match},
-as applied to \tcode{x <=> y}
-results in an ambiguity
-or a function that is deleted or inaccessible from the operator function,
-or
+as applied to \tcode{x <=> y},
+does not result in a usable function, or
 
 \item
 the operator \tcode{@}

source/compatibility.tex

@@ -2004,6 +2004,37 @@
 };
 \end{codeblock}
 
+\rSec2[diff.cpp17.over]{\ref{over}: overloading}
+
+\diffref{over.match.oper}
+\change
+Equality and inequality expressions can now find
+reversed and rewritten candidates.
+\rationale
+Improve consistency of equality with three-way comparison
+and make it easier to write the full complement of equality operations.
+\effect
+Equality and inequality expressions between two objects of different types,
+where one is convertible to the other,
+could invoke a different operator.
+Equality and inequality expressions between two objects of the same type
+could become ambiguous.
+\begin{codeblock}
+struct A {
+  operator int() const;
+};
+
+bool operator==(A, int);        // \#1
+// \#2 is built-in candidate: \tcode{bool operator==(int, int);}
+// \#3 is built-in candidate: \tcode{bool operator!=(int, int);}
+
+int check(A x, A y) {
+  return (x == y) +             // ill-formed; previously well-formed
+    (10 == x) +                 // calls \#1, previously selected \#2
+    (10 != x);                  // calls \#1, previously selected \#3
+}
+\end{codeblock}
+
 \rSec2[diff.cpp17.temp]{\ref{temp}: templates}
 
 \diffref{temp.names}

source/overloading.tex

@@ -1071,13 +1071,15 @@
 that is not a function template specialization.
 \end{itemize}
 
+\item
+The rewritten candidate set is determined as follows:
+\begin{itemize}
 \item
 For the relational\iref{expr.rel} operators,
 the rewritten candidates include
 all member, non-member, and built-in candidates
-for the operator \tcode{<=>}
-for which the rewritten expression
-\tcode{(x <=> y) @ 0} is well-formed using that \tcode{operator<=>}.
+for the expression \tcode{x <=> y}.
+\item
 For the
 relational\iref{expr.rel} and
 three-way comparison\iref{expr.spaceship}
@@ -1086,29 +1088,27 @@
 a synthesized candidate,
 with the order of the two parameters reversed,
 for each member, non-member, and built-in candidate
-for the operator \tcode{<=>}
-for which the rewritten expression
-\tcode{0 @ (y <=> x)} is well-formed using that \tcode{operator<=>}.
+for the expression
+\tcode{y <=> x}.
+\item
 For the \tcode{!=} operator\iref{expr.eq},
 the rewritten candidates
 include all member, non-member, and built-in candidates
-for the operator \tcode{==}
-for which the rewritten expression \tcode{(x == y)} is well-formed
-when contextually converted to \tcode{bool} using that operator \tcode{==}.
+for the expression \tcode{x == y}.
+\item
 For the equality operators,
 the rewritten candidates also include a synthesized candidate,
 with the order of the two parameters reversed,
-for each member, non-member, and built-in candidate for the operator \tcode{==}
-for which the rewritten expression \tcode{(y == x)} is well-formed
-when contextually converted to \tcode{bool} using that operator \tcode{==}.
+for each member, non-member, and built-in candidate
+for the expression \tcode{y == x}.
+\item
+For all other operators, the rewritten candidate set is empty.
+\end{itemize}
 \begin{note}
 A candidate synthesized from a member candidate has its implicit
 object parameter as the second parameter, thus implicit conversions
 are considered for the first, but not for the second, parameter.
 \end{note}
-In each case, rewritten candidates are not considered
-in the context of the rewritten expression.
-For all other operators, the rewritten candidate set is empty.
 \end{itemize}
 
 \pnum
@@ -1159,26 +1159,39 @@
 \end{example}
 
 \pnum
-If a rewritten candidate is selected by overload resolution
-for a relational or three-way comparison operator \tcode{@},
+If a rewritten \tcode{operator<=>} candidate
+is selected by overload resolution
+for an operator \tcode{@},
 \tcode{x @ y}
-is interpreted as the rewritten expression:
+is interpreted as
 \tcode{0 @ (y <=> x)}
 if the selected candidate is a synthesized candidate
 with reversed order of parameters,
 or \tcode{(x <=> y) @ 0} otherwise,
 using the selected rewritten \tcode{operator<=>} candidate.
-If a rewritten candidate is selected by overload resolution
-for a \tcode{!=} operator,
-\tcode{x != y} is interpreted as \tcode{(y == x) ?\ false :\ true}
-if the selected candidate is a synthesized candidate
-with reversed order of parameters, or
-\tcode{(x == y) ?\ false :\ true} otherwise,
-using the selected rewritten \tcode{operator==} candidate.
-If a rewritten candidate is selected by overload resolution
-for an \tcode{==} operator,
-\tcode{x == y} is interpreted as \tcode{(y == x) ?\ true :\ false}
-using the selected rewritten \tcode{operator==} candidate.
+Rewritten candidates for the operator \tcode{@}
+are not considered in the context of the resulting expression.
+
+\pnum
+If a rewritten \tcode{operator==} candidate
+is selected by overload resolution
+for an operator \tcode{@},
+its return type shall be \cv{} \tcode{bool}, and
+\tcode{x @ y} is interpreted as:
+\begin{itemize}
+\item
+if \tcode{@} is \tcode{!=}
+and the selected candidate is a synthesized candidate
+with reversed order of parameters,
+\tcode{!(y == x)},
+\item
+otherwise, if \tcode{@} is \tcode{!=},
+\tcode{!(x == y)},
+\item
+otherwise (when \tcode{@} is \tcode{==}),
+\tcode{y == x},
+\end{itemize}
+in each case using the selected rewritten \tcode{operator==} candidate.
 
 \pnum
 If a built-in candidate is selected by overload resolution, the