Merge 2022-11 CWG Motion 12

P2718R0 Wording for P2644R1 Fix for Range-based for Loop

Author: tkoeppe

source/basic.tex

@@ -4317,7 +4317,7 @@
 \pnum
 \indextext{initializer!temporary and declarator}%
 \indextext{temporary!order of destruction of}%
-There are three contexts in which temporaries are destroyed at a different
+There are four contexts in which temporaries are destroyed at a different
 point than the end of the full-expression.
 The first context is when a default constructor is called to initialize
 an element of an array with no corresponding initializer\iref{dcl.init}.
@@ -4438,14 +4438,24 @@
 \end{itemize}
 
 \pnum
-The destruction of a temporary whose lifetime is not extended by being
-bound to a reference is sequenced before the destruction of every
+The fourth context is when a temporary object other than a function parameter object
+is created in the \grammarterm{for-range-initializer} of a range-based \keyword{for} statement.
+If such a temporary object would otherwise be destroyed
+at the end of the \grammarterm{for-range-initializer} full-expression,
+the object persists for the lifetime of the reference
+initialized by the \grammarterm{for-range-initializer}.
+
+\pnum
+The destruction of a temporary whose lifetime is not extended
+beyond the full-expression in which it was created
+is sequenced before the destruction of every
 temporary which is constructed earlier in the same full-expression.
-If the lifetime of two or more temporaries to which references are bound ends
-at the same point,
+If the lifetime of two or more temporaries
+with lifetimes extending beyond the full-expressions in which they were created
+ends at the same point,
 these temporaries are destroyed at that point in the reverse order of the
 completion of their construction.
-In addition, the destruction of temporaries bound to references shall
+In addition, the destruction of such temporaries shall
 take into account the ordering of destruction of objects with static, thread, or
 automatic storage duration\iref{basic.stc.static,basic.stc.thread,basic.stc.auto};
 that is, if

source/compatibility.tex

@@ -69,6 +69,29 @@
                         // now equivalent to \tcode{arr.operator[](1, 2)} or ill-formed
 \end{codeblock}
 
+\rSec2[diff.cpp20.stmt]{\ref{stmt.stmt}: statements}
+
+\diffref{stmt.ranged}
+\change
+The lifetime of temporary objects in the \grammarterm{for-range-initializer}
+is extended until the end of the loop\iref{class.temporary}.
+\rationale
+Improve usability of the range-based \keyword{for} statement.
+\effect
+Destructors of some temporary objects are invoked later.
+For example:
+\begin{codeblock}
+void f() {
+  std::vector<int> v = { 42, 17, 13 };
+  std::mutex m;
+
+  for (int x :
+       static_cast<void>(std::lock_guard<std::mutex>(m)), v) {  // lock released in \CppXX
+    std::lock_guard<std::mutex> guard(m);                       // OK in \CppXX, now deadlocks
+  }
+}
+\end{codeblock}
+
 \rSec2[diff.cpp20.dcl]{\ref{dcl.dcl}: declarations}
 
 \diffref{dcl.init.string}

source/statements.tex

@@ -704,14 +704,30 @@
 \end{note}
 \end{itemize}
 \end{itemize}
-
 \begin{example}
 \begin{codeblock}
 int array[5] = { 1, 2, 3, 4, 5 };
 for (int& x : array)
   x *= 2;
 \end{codeblock}
 \end{example}
+\begin{note}
+The lifetime of some temporaries in the \grammarterm{for-range-initializer}
+is extended to cover the entire loop\iref{class.temporary}.
+\end{note}
+\begin{example}
+\begin{codeblock}
+using T = std::list<int>;
+const T& f1(const T& t) { return t; }
+const T& f2(T t)        { return t; }
+T g();
+
+void foo() {
+  for (auto e : f1(g())) {}     // OK, lifetime of return value of \tcode{g()} extended
+  for (auto e : f2(g())) {}     // undefined behavior
+}
+\end{codeblock}
+\end{example}
 
 \pnum
 In the \grammarterm{decl-specifier-seq} of a \grammarterm{for-range-declaration},