Merge 2019-02 CWG Motion 9

zygoloid · web-flow · commit f29e482d55d8 · 2019-03-11T17:27:33.000-07:00
P0960r3 Allow initializing aggregates from a parenthesized list of values Fixes #2689.
diff --git a/source/basic.tex b/source/basic.tex
@@ -3925,6 +3925,12 @@
 \item A temporary object bound to a reference parameter in a function call\iref{expr.call}
 persists until the completion of the full-expression containing the call.
 
+\item A temporary object bound to a reference element of
+an aggregate of class type initialized from
+a parenthesized \grammarterm{expression-list}\iref{dcl.init}
+persists until the completion of the full-expression
+containing the \grammarterm{expression-list}.
+
 \item The lifetime of a temporary bound to the returned value in a function \tcode{return} statement\iref{stmt.return} is not extended; the temporary is destroyed at the end of the full-expression in the \tcode{return} statement.
 
 \item A temporary bound to a reference in a \grammarterm{new-initializer}\iref{expr.new} persists until the completion of the full-expression containing the \grammarterm{new-initializer}.
diff --git a/source/declarations.tex b/source/declarations.tex
@@ -4370,7 +4370,23 @@
 and the initializer is a string literal, see~\ref{dcl.init.string}.
 \item If the initializer is \tcode{()}, the object is value-initialized.
 \item
-Otherwise, if the destination type is an array, the program is ill-formed.
+Otherwise, if the destination type is an array,
+the object is initialized as follows.
+Let $x_1$, $\dotsc$, $x_k$ be
+the elements of the \grammarterm{expression-list}.
+If the destination type is an array of unknown bound,
+it is defined as having $k$ elements.
+Let $n$ denote the array size after this potential adjustment.
+If $k$ is greater than $n$,
+the program is ill-formed.
+Otherwise, the $i^\text{th}$ array element is copy-initialized with
+$x_i$ for each $1 \leq i \leq k$, and
+value-initialized for each $k < i \leq n$.
+For each $1 \leq i < j \leq n$,
+every value computation and side effect associated with
+the initialization of the $i^\text{th}$ element of the array
+is sequenced before those associated with
+the initialization of the $j^\text{th}$ element.
 \item
 If the destination type is a (possibly cv-qualified) class type:
 
@@ -4390,12 +4406,60 @@
 constructors are considered.
 The applicable constructors
 are enumerated\iref{over.match.ctor}, and the best one is chosen
-through overload resolution\iref{over.match}.
-The constructor so selected
+through overload resolution\iref{over.match}. Then:
+\begin{itemize}
+\item
+If overload resolution is successful,
+the selected constructor
 is called to initialize the object, with the initializer
 expression or \grammarterm{expression-list} as its argument(s).
-If no constructor applies, or the overload resolution is
-ambiguous, the initialization is ill-formed.
+\item
+Otherwise, if no constructor is viable,
+the destination type is
+a (possibly cv-qualified) aggregate class \tcode{A}, and
+the initializer is a parenthesized \grammarterm{expression-list},
+the object is initialized as follows.
+Let $e_1$, $\dotsc$, $e_n$ be the elements of the aggregate\iref{dcl.init.aggr}.
+Let $x_1$, $\dotsc$, $x_k$ be the elements of the \grammarterm{expression-list}.
+If $k$ is greater than $n$, the program is ill-formed.
+The element $e_i$ is copy-initialized with
+$x_i$ for $1 \leq i \leq k$.
+The remaining elements are initialized with
+their default member initializers, if any, and
+otherwise are value-initialized.
+For each $1 \leq i < j \leq n$,
+every value computation and side effect
+associated with the initialization of $e_i$
+is sequenced before those associated with the initialization of $e_j$.
+\begin{note}
+By contrast with direct-list-initialization,
+narrowing conversions\iref{dcl.init.list} are permitted,
+designators are not permitted,
+a temporary object bound to a reference
+does not have its lifetime extended\iref{class.temporary}, and
+there is no brace elision.
+\begin{example}
+\begin{codeblock}
+struct A {
+  int a;
+  int&& r;
+};
+
+int f();
+int n = 10;
+
+A a1{1, f()};                   // OK, lifetime is extended
+A a2(1, f());                   // well-formed, but dangling reference
+A a3{1.0, 1};                   // error: narrowing conversion
+A a4(1.0, 1);                   // well-formed, but dangling reference
+A a5(1.0, std::move(n));        // OK
+\end{codeblock}
+\end{example}
+\end{note}
+\item
+Otherwise, the initialization is ill-formed.
+\end{itemize}
+
 \item
 Otherwise (i.e., for the remaining copy-initialization cases),
 user-defined conversions that can convert from the
diff --git a/source/preprocessor.tex b/source/preprocessor.tex
@@ -1475,6 +1475,7 @@
 \endhead
 \defnxname{cpp_aggregate_bases}                   & \tcode{201603L} \\ \rowsep
 \defnxname{cpp_aggregate_nsdmi}                   & \tcode{201304L} \\ \rowsep
+\defnxname{cpp_aggregate_paren_init}              & \tcode{201902L} \\ \rowsep
 \defnxname{cpp_alias_templates}                   & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_aligned_new}                       & \tcode{201606L} \\ \rowsep
 \defnxname{cpp_attributes}                        & \tcode{200809L} \\ \rowsep
