Merge 2024-11 CWG Motion 1

P3524R0 Core Language Working Group "ready" Issues

Author: tkoeppe

source/basic.tex

@@ -511,7 +511,8 @@
 either:
 \begin{itemize}
 \item the intervening scope is a block scope, or
-\item the intervening scope is the function parameter scope of a \grammarterm{lambda-expression}, or
+\item the intervening scope is the function parameter scope of
+a \grammarterm{lambda-expression} or \grammarterm{requires-expression}, or
 \item the intervening scope is the lambda scope of
 a \grammarterm{lambda-expression}
 that has a \grammarterm{simple-capture}
@@ -3115,7 +3116,9 @@
 \end{note}
 
 \pnum
-A \defn{memory location} is either an object of scalar type that is not a bit-field
+A \defn{memory location} is
+the storage occupied by the object representation of
+either an object of scalar type that is not a bit-field
 or a maximal sequence of adjacent bit-fields all having nonzero width.
 \begin{note}
 Various
@@ -5515,7 +5518,7 @@
 A pointer value $P$ is
 \indextext{value!valid in the context of an evaluation}%
 \defn{valid in the context of} an evaluation $E$
-if $P$ is a null pointer value, or
+if $P$ is a pointer to function or a null pointer value, or
 if it is a pointer to or past the end of an object $O$ and
 $E$ happens before the end of the duration of the region of storage for $O$.
 If a pointer value $P$ is used in an evaluation $E$ and
@@ -6037,13 +6040,30 @@
 \end{note}
 The value computations of the operands of an
 operator are sequenced before the value computation of the result of the
-operator. If a
+operator.
+The behavior is undefined if
+\begin{itemize}
+\item
 \indextext{side effects}%
-side effect on a memory location\iref{intro.memory} is unsequenced
-relative to either another side effect on the same memory location or
+a side effect on a memory location\iref{intro.memory} or
+\item
+starting or ending the lifetime of an object in a memory location
+\end{itemize}
+is unsequenced relative to
+\begin{itemize}
+\item
+another side effect on the same memory location,
+\item
+starting or ending the lifetime of an object occupying storage that
+overlaps with the memory location, or
+\item
 a value computation using the value of any object in the same memory location,
-and they are not potentially concurrent\iref{intro.multithread},
-the behavior is undefined.
+\end{itemize}
+and the two evaluations are not potentially concurrent\iref{intro.multithread}.
+\begin{note}
+Starting the lifetime of an object in a memory location can end the lifetime of
+objects in other memory locations\iref{basic.life}.
+\end{note}
 \begin{note}
 The next subclause imposes similar, but more complex restrictions on
 potentially concurrent computations.
@@ -6057,6 +6077,9 @@
   i = i++ + 1;                  // the value of \tcode{i} is incremented
   i = i++ + i;                  // undefined behavior
   i = i + 1;                    // the value of \tcode{i} is incremented
+
+  union U { int x, y; } u;
+  (u.x = 1, 0) + (u.y = 2, 0);  // undefined behavior
 }
 \end{codeblock}
 \end{example}
@@ -6067,8 +6090,13 @@
 the postfix expression designating the called function
 are sequenced before every expression or statement
 in the body of the called function.
-For each function invocation or
-evaluation of an \grammarterm{await-expression} \placeholder{F},
+For each
+\begin{itemize}
+\item function invocation,
+\item evaluation of an \grammarterm{await-expression}\iref{expr.await}, or
+\item evaluation of a \grammarterm{throw-expression}\iref{expr.throw}
+\end{itemize}
+\placeholder{F},
 each evaluation that does not occur within \placeholder{F} but
 is evaluated on the same thread and as part of the same signal handler (if any)
 is either sequenced before all evaluations that occur within \placeholder{F}
@@ -6167,9 +6195,21 @@
 \end{note}
 
 \pnum
-Two expression evaluations \defn{conflict} if one of them modifies\iref{defns.access} a memory
-location\iref{intro.memory} and the other one reads or modifies the same
-memory location.
+Two expression evaluations \defn{conflict} if one of them
+\begin{itemize}
+\item
+modifies\iref{defns.access} a memory location\iref{intro.memory} or
+\item
+starts or ends the lifetime of an object in a memory location
+\end{itemize}
+and the other one
+\begin{itemize}
+\item
+reads or modifies the same memory location or
+\item
+starts or ends the lifetime of an object occupying storage that
+overlaps with the memory location.
+\end{itemize}
 \begin{note}
 A modification can still conflict
 even if it does not alter the value of any bits.

source/classes.tex

@@ -3331,13 +3331,13 @@
 
 \pnum
 \indextext{nested class!local class}%
-If class \tcode{X} is a local class, a nested class \tcode{Y} may be
-declared in class \tcode{X} and later defined in the definition of class
-\tcode{X} or be later defined in the same scope as the definition of
-class \tcode{X}.
 \indextext{restriction!local class}%
 A class nested within
 a local class is a local class.
+A member of a local class \tcode{X} shall be
+declared only in the definition of \tcode{X} or,
+if the member is a nested class,
+in the nearest enclosing block scope of \tcode{X}.
 
 \pnum
 \indextext{restriction!static member local class}%
@@ -6178,60 +6178,62 @@
 \indextext{constructor!copy!elision}%
 \indextext{constructor!move!elision}%
 When certain criteria are met, an implementation is
-allowed to omit the copy/move construction of a class object,
-even if the constructor selected for the copy/move operation and/or the
+allowed to omit the creation of a class object from
+a source object of the same type (ignoring cv-qualification),
+even if the selected constructor and/or the
 destructor for the object have
 \indextext{side effects}%
 side effects.  In such cases, the
 implementation treats the source and target of the
-omitted copy/move operation as simply two different ways of
+omitted initialization as simply two different ways of
 referring to the same object. If the first parameter of the
 selected constructor is an rvalue reference to the object's type,
 the destruction of that object occurs when the target would have been destroyed;
 otherwise, the destruction occurs at the later of the times when the
 two objects would have been destroyed without the
 optimization.
-\begin{footnote}
+\begin{note}
 Because only one object is destroyed instead of two,
-and one copy/move constructor
-is not executed, there is still one object destroyed for each one constructed.
-\end{footnote}
-This elision of copy/move operations, called
+and the creation of one object is omitted,
+there is still one object destroyed for each one constructed.
+\end{note}
+This elision of object creation, called
 \indexdefn{copy elision|see{constructor, copy, elision}}%
 \indexdefn{elision!copy|see{constructor, copy, elision}}%
 \indexdefn{constructor!copy!elision}\indexdefn{constructor!move!elision}\term{copy elision},
 is permitted in the
 following circumstances (which may be combined to
 eliminate multiple copies):
 \begin{itemize}
-\item in a \tcode{return} statement in a function with a class return type,
+\item in a \tcode{return} statement\iref{stmt.return} in
+a function with a class return type,
 when the \grammarterm{expression} is the name of a non-volatile
-object with automatic storage duration (other than a function parameter or a variable
+object $o$ with automatic storage duration (other than a function parameter or a variable
 introduced by the \grammarterm{exception-declaration} of a
-\grammarterm{handler}\iref{except.handle})
-with the same type (ignoring cv-qualification) as
-the function return type, the copy/move operation can be
-omitted by constructing the object directly
-into the function call's return object
+\grammarterm{handler}\iref{except.handle}),
+the copy-initialization of the result object can be
+omitted by constructing $o$ directly
+into the function call's result object;
 
 \item in a \grammarterm{throw-expression}\iref{expr.throw}, when the operand
-is the name of a non-volatile object with automatic storage duration
-(other than a function or catch-clause parameter)
+is the name of a non-volatile object $o$ with automatic storage duration
+(other than a function parameter or
+a variable introduced by
+the \grammarterm{exception-declaration} of a \grammarterm{handler})
 that belongs to a scope that does not contain
 the innermost enclosing \grammarterm{compound-statement}
 associated with a \grammarterm{try-block} (if there is one),
-the copy/move operation can be omitted by
-constructing the object directly into the exception object
+the copy-initialization of the exception object can be omitted by
+constructing $o$ directly into the exception object;
 
 \item in a coroutine\iref{dcl.fct.def.coroutine}, a copy of a coroutine parameter
 can be omitted and references to that copy replaced with references to the
 corresponding parameter if the meaning of the program will be unchanged except for
-the execution of a constructor and destructor for the parameter copy object
+the execution of a constructor and destructor for the parameter copy object;
 
 \item when the \grammarterm{exception-declaration} of a
-\grammarterm{handler}\iref{except.handle} declares an object of the same
-type (except for cv-qualification) as the exception
-object\iref{except.throw}, the copy operation can be omitted by treating
+\grammarterm{handler}\iref{except.handle} declares an object $o$,
+the copy-initialization of $o$ can be omitted by treating
 the \grammarterm{exception-declaration} as an alias for the exception
 object if the meaning of the program will be unchanged except for the execution
 of constructors and destructors for the object declared by the

source/compatibility.tex

@@ -122,6 +122,38 @@
 \end{codeblock}
 \end{example}
 
+\diffref{temp.deduct.call}
+\change
+Template argument deduction from overload sets succeeds in more cases.
+\rationale
+Allow consideration of constraints to disambiguate overload sets
+used as parameters in function calls.
+\effect
+Valid \CppXXIII{} code may become ill-formed.
+\begin{example}
+\begin{codeblock}
+template <typename T>
+void f(T &&, void (*)(T &&));
+
+void g(int &);              // \#1
+inline namespace A {
+  void g(short &&);         // \#2
+}
+inline namespace B {
+  void g(short &&);         // \#3
+}
+
+void q() {
+  int x;
+  f(x, g);          // ill-formed; previously well-formed, deducing \tcode{T = int\&}
+}
+\end{codeblock}
+There is no change to the applicable deduction rules for
+the individual \tcode{g} candidates:
+Type deduction from \#1 does not succeed;
+type deductions from \#2 and \#3 both succeed.
+\end{example}
+
 \rSec2[diff.cpp23.library]{\ref{library}: library introduction}
 
 \diffref{headers}

source/declarations.tex

@@ -3641,7 +3641,7 @@
 If the
 \grammarterm{parameter-declaration-clause}
 is empty, the function takes no arguments.
-A parameter list consisting of a single unnamed parameter of
+A parameter list consisting of a single unnamed non-object parameter of
 non-dependent type \keyword{void} is equivalent to an empty parameter
 list.
 \indextext{parameter!\idxcode{void}}%
@@ -5810,6 +5810,10 @@
   return x;
 }
 constexpr int z = f();              // error: not a constant expression
+
+typedef int *A[3];                  // array of 3 pointer to \tcode{int}
+typedef const int *const CA[3];     // array of 3 const pointer to \tcode{const int}
+ACPC &&r = AP{};                    // binds directly
 \end{codeblock}
 \end{example}
 
@@ -9011,6 +9015,7 @@
 
 \rSec2[dcl.attr.assume]{Assumption attribute}
 
+\pnum
 The \grammarterm{attribute-token} \tcode{assume} may be applied to a null statement;
 such a statement is an \defn{assumption}.
 An \grammarterm{attribute-argument-clause} shall be present and
@@ -9023,7 +9028,10 @@
 If the converted expression would evaluate to \tcode{true}
 at the point where the assumption appears,
 the assumption has no effect.
-Otherwise, the behavior is undefined.
+Otherwise,
+evaluation of the assumption has runtime-undefined behavior.
+
+\pnum
 \begin{note}
 The expression is potentially evaluated\iref{basic.def.odr}.
 The use of assumptions is intended to allow implementations
@@ -9037,6 +9045,8 @@
 if an implementation does not attempt to deduce
 any such information from assumptions.
 \end{note}
+
+\pnum
 \begin{example}
 \begin{codeblock}
 int divide_by_32(int x) {
@@ -9530,8 +9540,9 @@
 translation unit, the program is ill-formed, no diagnostic required.
 
 \pnum
-If a function \tcode{f} is called where \tcode{f} was previously declared with the \tcode{noreturn}
-attribute and \tcode{f} eventually returns, the behavior is undefined.
+If a function \tcode{f} is invoked where \tcode{f} was previously declared with the \tcode{noreturn}
+attribute and that invocation eventually returns,
+the behavior is runtime-undefined.
 \begin{note}
 The function can
 terminate by throwing an exception.