Merge 2019-02 CWG Motion 1

P1358R0 Core Language Working Group "ready" Issues (Kona 2019)

After consulting with CWG, the resolution for CWG2331 was not applied and has been returned to CWG for reconsideration.

Fixes #2681.

Author: zygoloid

source/basic.tex

@@ -2802,24 +2802,26 @@
 \indextext{object lifetime|(}%
 The \defn{lifetime} of an object or reference is a runtime property of the
 object or reference.
-An object is said to have \defnadj{non-vacuous}{initialization} if it is of a class or
-array type and it or one of its subobjects is initialized by a constructor
-other than a trivial default constructor. \begin{note} Initialization by a
-trivial copy/move constructor is non-vacuous initialization. \end{note}
+\indextext{initialization!vacuous}%
+A variable is said to have \defn{vacuous initialization}
+if it is default-initialized and,
+if it is of class type or a (possibly multi-dimensional) array thereof,
+that class type has a trivial default constructor.
 The lifetime of an object of type \tcode{T} begins when:
 \begin{itemize}
-\item storage with the proper alignment and size for type \tcode{T} is
-obtained, and
-\item if the object has non-vacuous initialization, its initialization is complete,
+\item storage with the proper alignment and size
+  for type \tcode{T} is obtained, and
+\item its initialization (if any) is complete
+  (including vacuous initialization)\iref{dcl.init},
 \end{itemize}
 except that if the object is a union member or subobject thereof,
 its lifetime only begins if that union member is the
 initialized member in the union~(\ref{dcl.init.aggr}, \ref{class.base.init}),
 or as described in \ref{class.union}.
 The lifetime of an object \placeholder{o} of type \tcode{T} ends when:
 \begin{itemize}
-\item if \tcode{T} is a class type with a non-trivial
-destructor\iref{class.dtor}, the destructor call starts, or
+\item if \tcode{T} is a non-class type, the object is destroyed, or
+\item if \tcode{T} is a class type, the destructor call starts, or
 \item the storage which the object occupies is released,
 or is reused by an object that is not nested within \placeholder{o}\iref{intro.object}.
 \end{itemize}
@@ -5974,26 +5976,25 @@
 \indextext{program!termination|(}%
 \indextext{object!destructor static}%
 \indextext{\idxcode{main} function!return from}%
-Destructors\iref{class.dtor} for initialized objects
-(that is, objects whose lifetime\iref{basic.life} has begun)
-with static storage duration,
-and functions registered with \tcode{std::atexit},
+Constructed 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
 \indextext{\idxcode{exit}}%
 \indexlibrary{\idxcode{exit}}%
 \tcode{std::exit}\iref{support.start.term}.
 The call to \tcode{std::exit} is sequenced before
-the invocations of the destructors and the registered functions.
+the destructions and the registered functions.
 \begin{note}
 Returning from \tcode{main} invokes \tcode{std::exit}\iref{basic.start.main}.
 \end{note}
 
 \pnum
-Destructors for initialized objects with thread storage duration within a given thread
-are called as a result of returning from the initial function of that thread and as a
+Constructed 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 completions of the destructors for all initialized objects with thread storage
-duration within that thread strongly happen before the initiation of the destructors of
+The destruction of all constructed objects with thread storage
+duration within that thread strongly happens before destroying
 any object with static storage duration.
 
 \pnum

source/classes.tex

@@ -2120,7 +2120,6 @@
 \tcode{virtual}.
 
 \pnum
-A destructor is used to destroy objects of its class type.
 \indextext{restriction!destructor}%
 The address of a destructor shall not be taken.
 \indextext{\idxcode{const}!destructor and}%
@@ -6506,12 +6505,12 @@
 \begin{note} There cannot be a move from the exception object because it is
 always an lvalue.  \end{note}
 \end{itemize}
-Copy elision is required
+Copy elision is not permitted
 where an expression is evaluated in a context
 requiring a constant expression\iref{expr.const}
 and in constant initialization\iref{basic.start.static}.
 \begin{note}
-Copy elision might not be performed
+Copy elision might be performed
 if the same expression
 is evaluated in another context.
 \end{note}

source/declarations.tex

@@ -810,7 +810,7 @@
 each of its parameter types shall be a literal type;
 
 \item
-its \grammarterm{function-body} shall not contain
+its \grammarterm{function-body} shall not enclose\iref{stmt.stmt}
 \begin{itemize}
 \item an \grammarterm{asm-definition},
 \item a \tcode{goto} statement,
@@ -822,7 +822,7 @@
 \end{itemize}
 \begin{note}
 A \grammarterm{function-body} that is \tcode{= delete} or \tcode{= default}
-contains none of the above.
+encloses none of the above.
 \end{note}
 \end{itemize}
 
@@ -955,7 +955,7 @@
 a call to a constexpr
 function can appear in a constant expression\iref{expr.const} and
 \item
-copy elision is mandatory in a constant expression\iref{class.copy.elision}.
+copy elision is not performed in a constant expression\iref{class.copy.elision}.
 \end{itemize}
 
 \pnum
@@ -1310,8 +1310,11 @@
 \opt{\tcode{typename}} \opt{\grammarterm{nested-name-specifier}} \grammarterm{template-name}
 is a placeholder for
 a deduced class type\iref{dcl.type.class.deduct}.
-The \grammarterm{template-name} shall name a class template
-that is not an injected-class-name.
+The \grammarterm{template-name} shall name a class template.
+\begin{note}
+An injected-class-name is never interpreted as a \grammarterm{template-name}
+in contexts where class template argument deduction would be performed\iref{temp.local}.
+\end{note}
 The other
 \grammarterm{simple-type-specifier}{s}
 specify either a previously-declared type, a type determined from an
@@ -4469,13 +4472,18 @@
 \pnum
 An object whose initialization has completed
 is deemed to be constructed,
-even if no constructor of the object's class
+even if the object is of non-class type or
+no constructor of the object's class
 is invoked for the initialization.
 \begin{note}
 Such an object might have been value-initialized
 or initialized by aggregate initialization\iref{dcl.init.aggr}
 or by an inherited constructor\iref{class.inhctor.init}.
 \end{note}
+Destroying an object of class type invokes the destructor of the class.
+Destroying a scalar type has no effect other than
+ending the lifetime of the object\iref{basic.life}.
+Destroying an array destroys each element in reverse subscript order.
 
 \pnum
 A declaration that specifies the initialization of a variable,
@@ -4804,7 +4812,7 @@
 \end{example}
 
 \pnum
-If a reference member is initialized from its default member initializer
+If a member has a default member initializer
 and a potentially-evaluated subexpression thereof is an aggregate
 initialization that would use that default member initializer,
 the program is ill-formed.
@@ -4817,6 +4825,10 @@
     const A& a2 { A{} };        // error
   };
   A a{a,a};                     // OK
+
+  struct B {
+    int n = B{}.n;              // error
+  };
 \end{codeblock}
 \end{example}
 
@@ -5166,29 +5178,17 @@
 Given types ``\cvqual{cv1} \tcode{T1}'' and ``\cvqual{cv2} \tcode{T2}'',
 ``\cvqual{cv1} \tcode{T1}'' is \defn{reference-related} to
 ``\cvqual{cv2} \tcode{T2}'' if
-\tcode{T1} is the same type as \tcode{T2}, or
+\tcode{T1} is similar\iref{conv.qual} to \tcode{T2}, or
 \tcode{T1} is a base class of \tcode{T2}.
 ``\cvqual{cv1} \tcode{T1}'' is \defn{reference-compatible}
 with ``\cvqual{cv2} \tcode{T2}'' if
-\begin{itemize}
-\item \tcode{T1} is reference-related to \tcode{T2}, or
-\item \tcode{T2} is ``\tcode{noexcept} function'' and \tcode{T1} is ``function'',
-where the function types are otherwise the same,
-\end{itemize}
-and
-\cvqual{cv1}
-is the same cv-qualification as, or greater cv-qualification than,
-\cvqual{cv2}.
-In all cases where the reference-related or reference-compatible relationship
-of two types is used to establish the validity of a reference binding, and
-\tcode{T1}
-is a base class of
-\tcode{T2},
-a program that necessitates such a binding is ill-formed if
-\tcode{T1}
-is an inaccessible\iref{class.access} or ambiguous\iref{class.member.lookup}
-base class of
-\tcode{T2}.
+a prvalue of type ``pointer to \cvqual{cv2} \tcode{T2}'' can be converted to
+the type ``pointer to \cvqual{cv1} \tcode{T1}''
+via a standard conversion sequence\iref{conv}.
+In all cases where the reference-compatible relationship
+of two types is used to establish the validity of a reference binding and
+the standard conversion sequence would be ill-formed,
+a program that necessitates such a binding is ill-formed.
 
 \pnum
 A reference to type ``\cvqual{cv1} \tcode{T1}'' is initialized by
@@ -5586,9 +5586,7 @@
 
 \item Otherwise, if \tcode{T} is a reference type, a prvalue of the type
 referenced by \tcode{T} is generated.
-The prvalue initializes its result object by
-copy-list-initialization or direct-list-initialization,
-depending on the kind of initialization for the reference.
+The prvalue initializes its result object by copy-list-initialization.
 The prvalue is then used to direct-initialize the reference.
 \begin{note} As usual, the binding will fail and the program is ill-formed if
 the reference type is an lvalue reference to a non-const type. \end{note}
@@ -5606,6 +5604,10 @@
 const int& i1 = { 1 };              // OK
 const int& i2 = { 1.1 };            // error: narrowing
 const int (&iar)[2] = { 1, 2 };     // OK: \tcode{iar} is bound to temporary array
+
+struct A { } a;
+struct B { explicit B(const A&); };
+const B &b2{a};                     // error: cannot copy-list-initialize \tcode{B} temporary from \tcode{A}
 \end{codeblock}
 \end{example}
 
@@ -8780,15 +8782,19 @@
 
 \pnum
 The attribute may be applied to the declaration of a class,
-a \grammarterm{typedef-name}, a variable, a non-static data member,
+a \grammarterm{typedef-name},
+a variable (including a structured binding declaration),
+a non-static data member,
 a function, an enumeration, or an enumerator.
 
 \pnum
-\begin{note}
-For an entity marked \tcode{maybe_unused}, implementations
-should not emit a warning that the entity is unused, or
-that the entity is used despite the presence of the attribute.
-\end{note}
+For an entity marked \tcode{maybe_unused},
+implementations should not emit a warning
+that the entity or its structured bindings (if any)
+are used or unused.
+For a structured binding declaration not marked \tcode{maybe_unused},
+implementations should not emit such a warning unless
+all of its structured bindings are unused.
 
 \pnum
 A name or entity declared without the \tcode{maybe_unused} attribute

source/exceptions.tex

@@ -365,12 +365,12 @@
 \indextext{unwinding!stack}%
 As control passes from the point where an exception is thrown
 to a handler,
-destructors are invoked by a process, specified in this subclause, called
-\defn{stack unwinding}.
+objects with automatic storage duration are destroyed by a process,
+specified in this subclause, called \defn{stack unwinding}.
 
 \pnum
-The destructor is invoked for each automatic object of class type constructed,
-but not yet destroyed,
+Each object with automatic storage duration is destroyed if it has been
+constructed, but not yet destroyed,
 since the try block was entered.
 If an exception is thrown during the destruction of temporaries or
 local variables for a \tcode{return} statement\iref{stmt.return},
@@ -882,7 +882,9 @@
 is potentially-throwing if and only if
 any of the destructors
 for any of its potentially constructed subobjects
-is potentially throwing.
+is potentially-throwing or
+the destructor is virtual and the destructor of any virtual base class
+is potentially-throwing.
 
 \pnum
 The exception specification for an implicitly-declared assignment operator,

source/expressions.tex

@@ -973,7 +973,7 @@
 
 \pnum
 A prvalue of type ``pointer to \cv{} \tcode{D}'', where \tcode{D}
-is a class type, can be converted to a prvalue of type ``pointer to
+is a complete class type, can be converted to a prvalue of type ``pointer to
 \cv{} \tcode{B}'', where \tcode{B} is a base class\iref{class.derived}
 of \tcode{D}. If \tcode{B} is an
 inaccessible\iref{class.access} or
@@ -1005,8 +1005,8 @@
 A prvalue of type ``pointer to member of \tcode{B} of type \cv{}
 \tcode{T}'', where \tcode{B} is a class type, can be converted to
 a prvalue of type ``pointer to member of \tcode{D} of type \cv{}
-\tcode{T}'', where \tcode{D} is a derived class\iref{class.derived}
-of \tcode{B}. If \tcode{B} is an
+\tcode{T}'', where \tcode{D} is a complete class derived\iref{class.derived}
+from \tcode{B}. If \tcode{B} is an
 inaccessible\iref{class.access},
 ambiguous\iref{class.member.lookup}, or virtual\iref{class.mi} base
 class of \tcode{D}, or a base class of a virtual base class of
@@ -2201,7 +2201,11 @@
 
 \pnum
 A \grammarterm{lambda-expression} appearing in a default argument shall not
-implicitly or explicitly capture any entity.
+implicitly or explicitly capture any entity,
+except for an \grammarterm{init-capture} for which
+any full-expression in its \grammarterm{initializer}
+satisfies the constraints of an expression appearing in
+a default argument\iref{dcl.fct.default}.
 \begin{example}
 \begin{codeblock}
 void f2() {
@@ -2211,6 +2215,8 @@
   void g3(int = ([=]{ return i; })());          // ill-formed
   void g4(int = ([=]{ return 0; })());          // OK
   void g5(int = ([]{ return sizeof i; })());    // OK
+  void g6(int = ([x=1]{ return x; })();         // OK
+  void g7(int = ([x=i]{ return x; })();         // ill-formed
 }
 \end{codeblock}
 \end{example}
@@ -3713,7 +3719,8 @@
 \indextext{cast!derived class}%
 A prvalue of type ``pointer to \cvqual{cv1} \tcode{B}'', where \tcode{B}
 is a class type, can be converted to a prvalue of type ``pointer to
-\cvqual{cv2} \tcode{D}'', where \tcode{D} is a class derived\iref{class.derived}
+\cvqual{cv2} \tcode{D}'',
+where \tcode{D} is a complete class derived\iref{class.derived}
 from \tcode{B},
 if \cvqual{cv2} is the same cv-qualification as,
 or greater cv-qualification than, \cvqual{cv1}.
@@ -3732,8 +3739,9 @@
 \indextext{cast!pointer-to-member}%
 A prvalue of type ``pointer to member of \tcode{D} of type \cvqual{cv1}
 \tcode{T}'' can be converted to a prvalue of type ``pointer to member of
-\tcode{B} of type \cvqual{cv2} \tcode{T}'', where \tcode{B} is a base
-class\iref{class.derived} of \tcode{D},
+\tcode{B} of type \cvqual{cv2} \tcode{T}'', where
+\tcode{D} is a complete class type and
+\tcode{B} is a base class\iref{class.derived} of \tcode{D},
 if \cvqual{cv2} is the same cv-qualification
 as, or greater cv-qualification than, \cvqual{cv1}.\footnote{Function types
 (including those used in pointer-to-member-function
@@ -6505,7 +6513,7 @@
 requirements as detailed in this subclause; other contexts have different
 semantics depending on whether or not an expression satisfies these requirements.
 Expressions that satisfy these requirements,
-assuming that copy elision is performed,
+assuming that copy elision\iref{class.copy.elision} is not performed,
 are called
 \indexdefn{expression!constant}%
 \defnx{constant expressions}{constant expression}. \begin{note} Constant expressions can be evaluated

source/overloading.tex

@@ -1406,9 +1406,11 @@
 with ``\cvqual{cv2} \tcode{T2}'',
 are candidate functions. For direct-initialization, those explicit
 conversion functions that are not hidden within \tcode{S} and yield
-type ``lvalue reference to \cvqual{cv2} \tcode{T2}'' or ``\cvqual{cv2}
-\tcode{T2}'' or ``rvalue reference to \cvqual{cv2} \tcode{T2}'',
-respectively, where \tcode{T2} is the same type as \tcode{T} or can be
+type ``lvalue reference to \cvqual{cv2} \tcode{T2}''
+(when initializing an lvalue reference or an rvalue reference to function)
+or ``rvalue reference to \cvqual{cv2} \tcode{T2}''
+(when initializing an rvalue reference or an lvalue reference to function),
+where \tcode{T2} is the same type as \tcode{T} or can be
 converted to type \tcode{T} with a qualification
 conversion\iref{conv.qual}, are also candidate functions.