P2255R2 A type trait to detect reference binding to temporary

Author: jensmaurer

source/support.tex

@@ -674,6 +674,7 @@
 #define @\defnlibxname{cpp_lib_ranges_to_container}@               202202L // also in \libheader{ranges}
 #define @\defnlibxname{cpp_lib_ranges_zip}@                        202110L // also in \libheader{ranges}, \libheader{tuple}, \libheader{utility}
 #define @\defnlibxname{cpp_lib_raw_memory_algorithms}@             201606L // also in \libheader{memory}
+#define @\defnlibxname{cpp_lib_reference_from_temporary}@          202202L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_remove_cvref}@                      201711L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_result_of_sfinae}@                  201210L // also in \libheader{functional}, \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_robust_nonmodifying_seq_ops}@       201304L // also in \libheader{algorithm}

source/utilities.tex

@@ -872,6 +872,11 @@
 \begin{codeblock}
 !is_convertible_v<U1, T1> || !is_convertible_v<U2, T2>
 \end{codeblock}
+This constructor is defined as deleted if
+\tcode{reference_constructs_from_temporary_v<first_type, U1\&\&>}
+is \tcode{true} or
+\tcode{reference_constructs_from_temporary_v<second_type, U2\&\&>}
+is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryctor{pair}%
@@ -909,6 +914,12 @@
 !is_convertible_v<decltype(get<0>(@\exposid{FWD}@(p))), T1> ||
 !is_convertible_v<decltype(get<1>(@\exposid{FWD}@(p))), T2>
 \end{codeblock}
+The constructor is defined as deleted if
+\begin{codeblock}
+reference_constructs_from_temporary_v<first_type, decltype(get<0>(@\exposid{FWD}@(p)))> ||
+reference_constructs_from_temporary_v<second_type, decltype(get<1>(@\exposid{FWD}@(p)))>
+\end{codeblock}
+is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryctor{pair}%
@@ -936,6 +947,11 @@
 \tcode{std::forward<U>(x)}.) This form of construction, whereby constructor
 arguments for \tcode{first} and \tcode{second} are each provided in a separate
 \tcode{tuple} object, is called \defn{piecewise construction}.
+\begin{note}
+If a data member of \tcode{pair} is of reference type and
+its initialization binds it to a temporary object,
+the program is ill-formed\iref{class.base.init}.
+\end{note}
 \end{itemdescr}
 
 \indexlibrarymember{operator=}{pair}%
@@ -1731,6 +1747,11 @@
 \begin{codeblock}
 !conjunction_v<is_convertible<UTypes, Types>...>
 \end{codeblock}
+This constructor is defined as deleted if
+\begin{codeblock}
+(reference_constructs_from_temporary_v<Types, UTypes\&\&> || ...)
+\end{codeblock}
+is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryctor{tuple}%
@@ -1806,6 +1827,11 @@
 \begin{codeblock}
 !(is_convertible_v<decltype(get<I>(@\exposid{FWD}@(u))), Types> && ...)
 \end{codeblock}
+The constructor is defined as deleted if
+\begin{codeblock}
+(reference_constructs_from_temporary_v<Types, decltype(get<I>(@\exposid{FWD}@(u)))> || ...)
+\end{codeblock}
+is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryctor{tuple}%
@@ -1843,6 +1869,12 @@
 !is_convertible_v<decltype(get<0>(@\exposid{FWD}@(u))), @$\tcode{T}_0$@> ||
 !is_convertible_v<decltype(get<1>(@\exposid{FWD}@(u))), @$\tcode{T}_1$@>
 \end{codeblock}
+The constructor is defined as deleted if
+\begin{codeblock}
+reference_constructs_from_temporary_v<@$\tcode{T}_0$@, decltype(get<0>(@\exposid{FWD}@(u)))> ||
+reference_constructs_from_temporary_v<@$\tcode{T}_1$@, decltype(get<1>(@\exposid{FWD}@(u)))>
+\end{codeblock}
+is \tcode{true}.
 \end{itemdescr}
 
 \indexlibraryctor{tuple}%
@@ -2404,6 +2436,13 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\mandates
+If \tcode{tuple_size_v<remove_reference_t<Tuple>>} is 1,
+then
+\tcode{reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))>}
+is \tcode{false}.
+
 \pnum
 \effects
 Given the exposition-only function:
@@ -16393,6 +16432,11 @@
 if \tcode{R} is \cv{}~\keyword{void}, otherwise
 \tcode{\placeholdernc{INVOKE}(f, t$_1$, t$_2$, $\dotsc$, t$_N$)} implicitly converted
 to \tcode{R}.
+If
+\tcode{reference_converts_from_temporary_v<R, decltype(\placeholdernc{INVOKE}(f, t$_1$, t$_2$, $\dotsc$, t$_N$))>}
+is \tcode{true},
+\tcode{\placeholdernc{INVOKE}<R>(f, t$_1$, t$_2$, $\dotsc$, t$_N$)}
+is ill-formed.
 
 \pnum
 \indextext{call wrapper}%
@@ -19554,6 +19598,9 @@
 
   template<class T> struct has_unique_object_representations;
 
+  template<class T, class U> struct reference_constructs_from_temporary;
+  template<class T, class U> struct reference_converts_from_temporary;
+
   // \ref{meta.unary.prop.query}, type property queries
   template<class T> struct alignment_of;
   template<class T> struct rank;
@@ -19826,6 +19873,12 @@
   template<class T>
     inline constexpr bool has_unique_object_representations_v
       = has_unique_object_representations<T>::value;
+  template<class T, class U>
+    inline constexpr bool @\libglobal{reference_constructs_from_temporary_v}@
+      = reference_constructs_from_temporary<T, U>::value;
+  template<class T, class U>
+    inline constexpr bool @\libglobal{reference_converts_from_temporary_v}@
+      = reference_converts_from_temporary<T, U>::value;
 
   // \ref{meta.unary.prop.query}, type property queries
   template<class T>
@@ -20081,6 +20134,24 @@
 in the context of the corresponding definition
 notwithstanding the restrictions of~\ref{declval}.
 
+\pnum
+For the purpose of defining the templates in this subclause,
+let \tcode{\placeholdernc{VAL}<T>} for some type \tcode{T} be
+an expression defined as follows:
+\begin{itemize}
+\item
+If \tcode{T} is a reference or function type,
+\tcode{\placeholdernc{VAL}<T>} is an expression
+with the same type and value category as \tcode{declval<T>()}.
+\item
+Otherwise, \tcode{\placeholdernc{VAL}<T>} is a prvalue
+that initially has type \tcode{T}.
+\begin{note}
+If \tcode{T} is cv-qualified,
+the cv-qualification is subject to adjustment\iref{expr.type}.
+\end{note}
+\end{itemize}
+
 \begin{libreqtab3b}{Type property predicates}{meta.unary.prop}
 \\ \topline
 \lhdr{Template} &   \chdr{Condition}    &   \rhdr{Preconditions}    \\ \capsep
@@ -20492,6 +20563,25 @@
   \tcode{T} shall be a complete type, \cv{}~\keyword{void}, or
   an array of unknown bound. \\ \rowsep
 
+\indexlibraryglobal{reference_constructs_from_temporary}%
+\tcode{template<class T, class U>}\br
+  \tcode{struct reference_constructs_from_temporary;} &
+  \tcode{conjunction_v<is_reference<T>, is_constructible<T, U>>}
+  is \tcode{true}, and
+  the initialization \tcode{T t(\exposidnc{VAL}<U>);} binds \tcode{t} to
+  a temporary object whose lifetime is extended\iref{class.temporary}. &
+  \tcode{T} and \tcode{U} shall be
+  complete types, \cv{}~\keyword{void}, or arrays of unknown bound.  \\ \rowsep
+
+\indexlibraryglobal{reference_converts_from_temporary}%
+\tcode{template<class T, class U>}\br
+  \tcode{struct reference_converts_from_temporary;} &
+  \tcode{conjunction_v<is_reference<T>, is_convertible<U, T>>} is \tcode{true},
+  and the initialization \tcode{T t = \exposidnc{VAL}<U>;} binds \tcode{t} to
+  a temporary object whose lifetime is extended\iref{class.temporary}. &
+  \tcode{T} and \tcode{U} shall be
+  complete types, \cv{}~\keyword{void}, or arrays of unknown bound.  \\ \rowsep
+
 \end{libreqtab3b}
 
 \pnum