Merge 2025-11 CWG Motion 1

P3921R0 Core Language Working Group "ready" Issues for the November, 2025 meeting;
excluding issues 1670, 2917, 2923, 3005, 3043, 3044, 3045, 3048, 3053, 3061, 3063, 3074, 3082, 3084,
3089, 3092, 3093, 3094, 3095, 3098, 3099, 3101, 3108, 3109, 3110, 3113, 3114, 3115, 3117, and 3118.

Author: tkoeppe

source/basic.tex

@@ -954,7 +954,8 @@
 declaration,
 \grammarterm{parameter-declaration-clause},
 \grammarterm{statement},
-\grammarterm{handler}, or
+\grammarterm{handler},
+\grammarterm{lambda-expression}, or
 contract assertion
 (as described in the following subclauses of \ref{basic.scope})
 appearing in another scope, which thereby contains $S$.
@@ -2887,8 +2888,8 @@
 \indextext{program}%
 \indextext{linking}%
 A \defn{program} consists of one or more translation units\iref{lex.phases}
-linked together. A translation unit consists
-of a sequence of declarations.
+that are translated and linked together.
+A translation unit consists of a sequence of declarations.
 
 \begin{bnf}
 \nontermdef{translation-unit}\br
@@ -3476,7 +3477,7 @@
 An object that is not a subobject of any other object is called a \defn{complete
 object}.
 If an object is created
-in storage associated with a member subobject or array element \placeholder{e}
+in storage associated with a subobject \placeholder{e}
 (which may or may not be within its lifetime),
 the created object
 is a subobject of \placeholder{e}'s containing object if
@@ -3486,8 +3487,12 @@
 \item
 the storage for the new object exactly overlays the storage location associated with \placeholder{e}, and
 \item
+\placeholder{e} is not a potentially-overlapping subobject, and
+\item
 the new object is of the same type as \placeholder{e} (ignoring cv-qualification).
 \end{itemize}
+In this case, \placeholder{e} and the created object are
+\defnadjx{corresponding direct}{subobjects}{subobject}.
 
 \pnum
 \indextext{object!providing storage for}%
@@ -3940,16 +3945,8 @@
   the pointer is used to access a non-static data member or call a
   non-static member function of the object, or
 \item
-  the pointer is implicitly converted\iref{conv.ptr} to a pointer
-  to a virtual base class, or
-\item
-  the pointer is used as the operand of a
-  \keyword{static_cast}\iref{expr.static.cast}, except when the conversion
-  is to pointer to \cv{}~\keyword{void}, or to pointer to \cv{}~\keyword{void}
-  and subsequently to pointer to
-  \cv{}~\keyword{char},
-  \cv{}~\tcode{\keyword{unsigned} \keyword{char}}, or
-  \cv{}~\tcode{std::byte}\iref{cstddef.syn}, or
+  the pointer is converted\iref{conv.ptr,expr.static.cast} to a pointer
+  to a virtual base class or to a base class thereof, or
 \item
   the pointer is used as the operand of a
   \keyword{dynamic_cast}\iref{expr.dynamic.cast}.
@@ -4012,22 +4009,24 @@
 \end{note}
 
 \pnum
-An object $o_1$ is \defn{transparently replaceable} by an object $o_2$ if
+An object $o_1$ is \defn{transparently replaceable} by an object $o_2$ if either
+\begin{itemize}
+\item
+$o_1$ and $o_2$ are complete objects for which:
 \begin{itemize}
+\item $o_1$ is not const,
 \item the storage that $o_2$ occupies exactly overlays
 the storage that $o_1$ occupied, and
-
 \item $o_1$ and $o_2$ are of the same type
-(ignoring the top-level cv-qualifiers), and
-
-\item $o_1$ is not a const, complete object, and
-
-\item neither $o_1$ nor $o_2$
-is a potentially-overlapping subobject\iref{intro.object}, and
+(ignoring the top-level cv-qualifiers), or
+\end{itemize}
 
-\item either $o_1$ and $o_2$ are both complete objects, or
-$o_1$ and $o_2$ are direct subobjects of objects $p_1$ and $p_2$, respectively,
-and $p_1$ is transparently replaceable by $p_2$.
+\item
+$o_1$ and $o_2$ are corresponding direct subobjects\iref{intro.object} for which:
+\begin{itemize}
+\item the complete object of $o_1$ is not const or
+\item $o_1$ is a mutable member subobject or a subobject thereof.
+\end{itemize}
 \end{itemize}
 
 \pnum
@@ -4812,7 +4811,9 @@
 \item
   a class member access\iref{expr.ref} using the \tcode{.} operator
   where the left operand is one of these expressions and
-  the right operand designates a non-static data member of non-reference type,
+  the right operand designates
+  a non-static data member\iref{class.mem.general} of non-reference type or
+  a direct base class relationship\iref{class.derived.general},
 \item
   a pointer-to-member operation\iref{expr.mptr.oper} using the \tcode{.*} operator
   where the left operand is one of these expressions and
@@ -4830,6 +4831,15 @@
   to a glvalue that refers
   to the object designated by the operand, or
   to its complete object or a subobject thereof,
+\item
+  an explicit type conversion (functional notation)\iref{expr.type.conv}
+  to a reference type whose initializer is a \grammarterm{braced-init-list}
+  where the reference is
+  \begin{itemize}
+  \item bound directly to the glvalue result of one of these expressions
+  (necessarily the sole element of the \grammarterm{braced-init-list}) or
+  \item bound to the result of a temporary materialization conversion,
+  \end{itemize}
 \item
   a conditional expression\iref{expr.cond} that is a glvalue
   where the second or third operand is one of these expressions, or
@@ -5592,20 +5602,23 @@
 is an incomplete type that cannot be completed; such a type has
 an empty set of values. It is used as the return
 type for functions that do not return a value.
-An expression of type \cv{}~\keyword{void} shall
-be used only as
+\begin{note}
+An expression of type \cv{}~\keyword{void} can be used as
 \begin{itemize}
 \item an expression statement\iref{stmt.expr},
 \item the expression in a \keyword{return} statement\iref{stmt.return}
 for a function with the return type \cv{}~\keyword{void},
 \item an operand of a comma expression\iref{expr.comma},
+\item the operand of a parenthesized expression\iref{expr.prim.paren},
+\item a requirement in a \grammarterm{requires-expression}\iref{expr.prim.req.general},
 \item the second or third operand of \tcode{?:}\iref{expr.cond},
 \item the operand of a \keyword{typeid} expression\iref{expr.typeid},
 \item the operand of a \keyword{noexcept} operator\iref{expr.unary.noexcept},
 \item the operand of a \keyword{decltype} specifier\iref{dcl.type.decltype}, or
 \item the operand of an explicit conversion to type
 \cv{}~\keyword{void}\iref{expr.type.conv,expr.static.cast,expr.cast}.
 \end{itemize}
+\end{note}
 
 \pnum
 The types denoted by \cv~\tcode{std::nullptr_t} are distinct types.
@@ -7170,8 +7183,13 @@
 \indextext{\idxcode{main} function!implementation-defined parameters to}%
 An implementation shall allow both
 \begin{itemize}
-\item a function of \tcode{()} returning \keyword{int} and
-\item a function of \tcode{(\keyword{int}}, pointer to pointer to \tcode{\keyword{char})} returning \keyword{int}
+\item
+an ``optionally \tcode{noexcept} function of
+\tcode{()} returning \keyword{int}'' and
+\item
+an ``optionally \tcode{noexcept} function of
+\tcode{(\keyword{int}}, pointer to pointer to \tcode{\keyword{char})}
+returning \keyword{int}''
 \end{itemize}
 \indextext{\idxcode{argc}}%
 \indextext{\idxcode{argv}}%
@@ -7489,7 +7507,7 @@
 \indextext{program!termination|(}%
 \indextext{object!destructor static}%
 \indextext{\idxcode{main} function!return from}%
-Constructed objects\iref{dcl.init}
+Constructed complete objects\iref{dcl.init}
 with static storage duration are destroyed
 and functions registered with \tcode{std::atexit}
 are called as part of a call to
@@ -7503,29 +7521,69 @@
 \end{note}
 
 \pnum
-Constructed objects with thread storage duration within a given thread
+Constructed complete objects with thread storage duration within a given thread
 are destroyed as a result of returning from the initial function of that thread and as a
 result of that thread calling \tcode{std::exit}.
-The destruction of all constructed objects with thread storage
-duration within that thread strongly happens before destroying
+The destruction of those constructed objects
+is sequenced before releasing the storage for
+any object with thread storage duration within that thread\iref{basic.stc.thread}.
+The destruction of those constructed objects
+strongly happens before destroying
 any object with static storage duration.
 
 \pnum
-If the completion of the constructor or dynamic initialization of an object with static
-storage duration strongly happens before that of another, the completion of the destructor
-of the second is sequenced before the initiation of the destructor of the first.
-If the completion of the constructor or dynamic initialization of an object with thread
-storage duration is sequenced before that of another, the completion of the destructor
-of the second is sequenced before the initiation of the destructor of the first.
+The destruction of a complete object with thread storage duration within a given thread
+and having constant destruction\iref{expr.const}
+is sequenced after the destruction of any other complete object
+with thread storage duration within the thread.
+The destruction of a complete object with static storage duration
+and having constant destruction
+is sequenced after the destruction of any other complete object
+with static storage duration
+and after any call to a function passed to \tcode{std::atexit}.
+The sequencing rules in the remainder of this subclause
+apply only to complete objects not having constant destruction.
+
+\pnum
+If the deemed construction\iref{dcl.init.general} of a complete object with static
+storage duration strongly happens before that of another, the completion of the destruction
+of the second is sequenced before the initiation of the destruction of the first.
+If the deemed construction of a complete object with thread
+storage duration is sequenced before that of another, the completion of the destruction
+of the second is sequenced before the initiation of the destruction of the first.
 If an object is
 initialized statically, the object is destroyed in the same order as if
-the object was dynamically initialized. For an object of array or class
-type, all subobjects of that object are destroyed before any block
-variable with static storage duration initialized during the construction
-of the subobjects is destroyed.
+the object was dynamically initialized.
 If the destruction of an object with static or thread storage duration
 exits via an exception,
 the function \tcode{std::terminate} is called\iref{except.terminate}.
+\begin{example}
+In the following program,
+the elements of \tcode{a} are destroyed,
+followed by \tcode{dt}, and
+finally by the two \tcode{Btemp} objects:
+\begin{codeblock}
+struct DTemp { ~DTemp(); };
+struct Temp {
+  ~Temp() {
+    static DTemp dt;
+  }
+};
+struct BTemp {
+  ~BTemp();
+};
+struct A {
+  const BTemp &tb;
+  ~A();
+};
+A a[] = { (Temp(), BTemp()), BTemp() };
+
+int main() {}
+\end{codeblock}
+If the array \tcode{a} were an object with automatic storage duration,
+the \tcode{Btemp} temporaries would be destroyed
+as each element of the array is destroyed\iref{class.temporary}.
+\end{example}
 
 \pnum
 If a function contains a block variable of static or thread storage duration that has been
@@ -7541,12 +7599,13 @@
 \pnum
 \indextext{\idxcode{atexit}}%
 \indexlibraryglobal{atexit}%
-If the completion of the initialization of an object with static storage
+If the deemed construction of a complete object with static storage
 duration strongly happens before a call to \tcode{std::atexit}~(see
 \libheader{cstdlib}, \ref{support.start.term}), the call to the function passed to
-\tcode{std::atexit} is sequenced before the call to the destructor for the object. If a
-call to \tcode{std::atexit} strongly happens before the completion of the initialization of
-an object with static storage duration, the call to the destructor for the
+\tcode{std::atexit} is sequenced before the initiation of the destruction of the object.
+If a call to \tcode{std::atexit} strongly happens before the deemed construction of
+a complete object with static storage duration,
+the completion of the destruction of the
 object is sequenced before the call to the function passed to \tcode{std::atexit}. If a
 call to \tcode{std::atexit} strongly happens before another call to \tcode{std::atexit}, the
 call to the function passed to the second \tcode{std::atexit} call is sequenced before

source/classes.tex

@@ -1704,7 +1704,8 @@
 and no virtual base classes\iref{class.mi}, and
 
 \item
-the constructor selected to copy/move each direct base class subobject is trivial, and
+the constructor selected to copy/move each direct base class subobject
+is a direct member of that base class and is trivial, and
 
 \item
 for each non-static data member of
@@ -1986,7 +1987,7 @@
 and no virtual base classes\iref{class.mi}, and
 
 \item the assignment operator selected to copy/move each direct
-base class subobject is trivial, and
+base class subobject is a direct member of that base class and is trivial, and
 
 \item
 for each non-static data member of
@@ -3356,7 +3357,8 @@
 \indextext{access control!anonymous \tcode{union}}%
 \indextext{restriction!anonymous \tcode{union}}%
 Each \grammarterm{member-declaration} in the \grammarterm{member-specification}
-of an anonymous union shall either define one or more public non-static data members or
+of an anonymous union shall define one or more public non-static data members,
+be an \grammarterm{empty-declaration}, or
 be a \grammarterm{static_assert-declaration}.
 Nested types, anonymous unions, and functions
 shall not be declared within an anonymous union.

source/compatibility.tex

@@ -3041,6 +3041,33 @@
 (for instance, via the C typedef in \libheaderref{stdbool.h})
 is ill-formed in \Cpp{}.
 
+\diffref{expr.unary.op}
+\change
+In certain contexts,
+taking the address of a dereferenced null or past-the-end pointer value
+is well-defined in C (and yields the original pointer value),
+but results in undefined behavior in \Cpp{}.
+\begin{example}
+\begin{codeblock}
+void f() {
+  char *p = nullptr;
+  char *p2 = &*p;       // well-defined in C, undefined behavior in \Cpp{}
+  char *p3 = &p[0];     // well-defined in C, undefined behavior in \Cpp{}
+  int a[5];
+  int *q = &a[5];       // well-defined in C, undefined behavior in \Cpp{}
+}
+\end{codeblock}
+\end{example}
+\rationale
+Consistent treatment of lvalues in \Cpp{}.
+\effect
+Well-formed and well-defined C code exhibits undefined behavior in \Cpp{}.
+\difficulty
+Syntactic transformation to pointer arithmetic
+and possible addition of a check for null pointer values.
+\howwide
+Occasionally.
+
 \diffref{expr.sizeof,expr.cast}
 \change
 In \Cpp{}, types can only be defined in declarations, not in expressions.\\
@@ -3165,6 +3192,24 @@
 
 \rSec2[diff.dcl]{\ref{dcl}: declarations}
 
+\diffref{dcl.pre}
+\change
+In \Cpp{}, no declaration of a variable can have \cv{} \tcode{void} type.
+In C, this is allowed, unless the declaration is a definition.
+\begin{example}
+\begin{codeblock}
+extern void x;  // valid C, invalid in \Cpp{}
+\end{codeblock}
+\end{example}
+\rationale
+Stricter type checking in \Cpp{}.
+\effect
+Deletion of semantically well-defined feature.
+\difficulty
+Syntactic transformation.
+\howwide
+Seldom.
+
 \diffref{dcl.stc}
 \change
 In \Cpp{}, the \keyword{static} or \keyword{extern} specifiers can only be applied to names of objects or functions.\\