[CWG Motion 6] P2953R5 Adding restrictions to defaulted assignment operator functions

source/compatibility.tex

@@ -1,6 +1,46 @@
 %!TEX root = std.tex
 \infannex{diff}{Compatibility}
 
+\rSec1[diff.cpp26]{\Cpp{} and ISO \CppXXVI{}}
+
+\rSec2[diff.cpp26.general]{General}
+
+\pnum
+\indextext{summary!compatibility with ISO \CppXXVI{}}%
+Subclause \ref{diff.cpp26} lists the differences between \Cpp{} and
+ISO \CppXXVI{},
+by the chapters of this document.
+
+\rSec2[diff.cpp26.dcl]{\ref{dcl}: declarations}
+
+\diffref{dcl.fct.def.default}
+\change
+It is no longer valid
+for explicitly defaulted assignment operators to have a
+\grammarterm{cv-qualifier-seq} or \tcode{\&\&} \grammarterm{ref-qualifier}, or
+for an explicitly defaulted move assignment operator
+to have a parameter of type ``reference to \tcode{T}''
+where \tcode{T} is cv-qualified.
+\rationale
+Removal of rarely-used and confusing feature.
+\effect
+A valid \CppXXVI{} program
+which uses these features on an explicitly defaulted assignment operator
+is ill-formed.
+\begin{example}
+\begin{codeblock}
+struct S {
+  S& operator=(const S&) && = default;          // ill-formed; previously well-formed
+};
+struct T {
+  T& operator=(const T&) const = default;       // ill-formed; previously well-formed but deleted
+};
+struct U {
+  U& operator=(const U&&) = default;            // ill-formed; previously well-formed but deleted
+};
+\end{codeblock}
+\end{example}
+
 \rSec1[diff.cpp23]{\Cpp{} and ISO \CppXXIII{}}
 
 \rSec2[diff.cpp23.general]{General}

source/declarations.tex

@@ -7005,41 +7005,39 @@
 \end{itemize}
 
 \pnum
-An explicitly defaulted special member function $\tcode{F}_1$
-is allowed to differ from
-the corresponding special member function $\tcode{F}_2$
-that would have been implicitly declared, as follows:
+The program is ill-formed
+if an explicitly defaulted special member function $\tcode{F}_1$
+of class \tcode{C}
+differs from the corresponding special member function $\tcode{F}_2$
+that would have been implicitly declared
+other than as follows:
 \begin{itemize}
 \item
-  $\tcode{F}_1$ and $\tcode{F}_2$ may have differing \grammarterm{ref-qualifier}{s};
+  if $\tcode{F}_1$ is an assignment operator,
+  it may have the \tcode{\&} \grammarterm{ref-qualifier};
 \item
-  if $\tcode{F}_2$ has an implicit object parameter of
-  type ``reference to \tcode{C}'',
-  $\tcode{F}_1$ may be an explicit object member function whose
-  explicit object parameter is of (possibly different) type ``reference to \tcode{C}'',
+  if $\tcode{F}_2$ is an assignment operator
+  (which has an implicit object parameter
+  of type ``lvalue reference to \tcode{C}''),
+  $\tcode{F}_1$ may have an explicit object parameter
+  of type ``lvalue reference to \tcode{C}'',
   in which case the type of $\tcode{F}_1$ would differ from the type of $\tcode{F}_2$
   in that the type of $\tcode{F}_1$ has an additional parameter;
 \item
-  $\tcode{F}_1$ and $\tcode{F}_2$ may have differing exception specifications; and
+  $\tcode{F}_1$ and $\tcode{F}_2$ may have differing exception specifications;
 \item
-  if $\tcode{F}_2$ has a non-object parameter of type \tcode{const C\&},
-  the corresponding non-object parameter of $\tcode{F}_1$ may be of
-  type \tcode{C\&}.
-\end{itemize}
-If the type of $\tcode{F}_1$ differs from the type of $\tcode{F}_2$ in a way
-other than as allowed by the preceding rules, then:
-\begin{itemize}
-\item
-  if $\tcode{F}_1$ is an assignment operator, and
-  the return type of $\tcode{F}_1$ differs from
-  the return type of $\tcode{F}_2$ or
-  $\tcode{F}_1${'s} non-object parameter type is not a reference,
-  the program is ill-formed;
-\item
-  otherwise, if $\tcode{F}_1$ is explicitly defaulted on its first declaration,
-  it is defined as deleted;
+  if $\tcode{F}_2$ has a non-object parameter
+  of type ``lvalue reference to const \tcode{C}'',
+  the corresponding non-object parameter of $\tcode{F}_1$ may be
+  of type ``lvalue reference to \tcode{C}''; and
 \item
-  otherwise, the program is ill-formed.
+  if $\tcode{F}_2$ has a non-object parameter
+  of type ``lvalue reference to \tcode{C}'',
+  the corresponding non-object parameter of $\tcode{F}_1$ may be
+  of type ``lvalue reference to const \tcode{C}'';
+  in this case only, $\tcode{F}_1$ is defined as deleted
+  if it is explicitly defaulted on its first declaration and
+  the program is ill-formed otherwise.
 \end{itemize}
 
 \pnum