Merge 2026-03 CWG Motion 9

P3726R2 Adjustments to Union Lifetime Rules

Author: tkoeppe

source/basic.tex

@@ -3694,23 +3694,71 @@
 the address of an unspecified byte of storage
 occupied by the complete object of that subobject.
 
+\pnum
+A \defnadj{union elemental}{subobject}
+\indextext{union!elemental subobject|see{subobject, union elemental}}
+is a direct member of a union or
+an element of an array that is a union elemental subobject.
+An \defnx{inactive union elemental subobject}{subobject!union elemental!inactive}
+\indextext{inactive union elemental subobject|see{subobject, union elemental, inactive}}
+\indextext{union!inactive elemental subobject|see{subobject, union elemental, inactive}}
+is a union elemental subobject that is not within its lifetime.
+
 \pnum
 The \defnx{constituent values}{constituent value} of an object $o$ are
 \begin{itemize}
 \item
 if $o$ has scalar type, the value of $o$;
 \item
 otherwise, the constituent values of any direct subobjects of $o$
-other than inactive union members.
+other than inactive union elemental subobjects.
 \end{itemize}
 The \defnx{constituent references}{constituent reference} of an object $o$ are
 \begin{itemize}
 \item
 any direct members of $o$ that have reference type, and
 \item
 the constituent references of any direct subobjects of $o$
-other than inactive union members.
+other than inactive union elemental subobjects.
 \end{itemize}
+\begin{example}
+\begin{codeblock}
+struct A {
+  struct X {
+    int i;
+    int j;
+  };
+
+  struct Y {
+    X x1;
+    X x2;
+  };
+
+  union {
+    int i;
+    int arr[4];
+    Y y;
+  };
+};
+
+constexpr A v1;         // OK, no constituent values
+constexpr A v2{.i=1};   // OK, the constituent values are \tcode{\{v2.i\}}
+constexpr A v3 = []{
+  A a;
+  std::start_lifetime(a.arr);   // OK, arr is now the active element of the union
+  new (&a.arr[1]) int(1);
+  a.arr[2] = 2;
+  return a;
+}();                    // OK, the constituent values are \tcode{\{v3.arr[1], v3.arr[2]\}}
+constexpr A v4 = []{
+  A a;
+  a.y.x1.i = 1;
+  a.y.x2.j = 2;
+  return a;
+}();                    // error: the constituent values include \tcode{v4.y.x1.j} and \tcode{v4.y.x2.i}
+                        // which have erroneous value
+\end{codeblock}
+\end{example}
 
 \pnum
 Some operations are described as

source/classes.tex

@@ -1337,18 +1337,7 @@
 \defnx{non-trivial}{constructor!default!non-trivial}.
 
 \pnum
-If a default constructor of a union-like class \tcode{X} is trivial,
-then for each union \tcode{U}
-that is either \tcode{X} or an anonymous union member of \tcode{X},
-if the first variant member, if any, of \tcode{U}
-has implicit-lifetime type\iref{basic.types.general},
-the default constructor of \tcode{X} begins the lifetime of that member
-if it is not the active member of its union.
-\begin{note}
-It is already the active member if \tcode{U} was value-initialized.
-\end{note}
-Otherwise,
-an implicitly-defined\iref{dcl.fct.def.default} default constructor performs the set of
+An implicitly-defined\iref{dcl.fct.def.default} default constructor performs the set of
 initializations of the class that would be performed by a user-written default
 constructor for that class with no
 \grammarterm{ctor-initializer}\iref{class.base.init} and an empty

source/memory.tex

@@ -88,6 +88,8 @@
     bool is_sufficiently_aligned(T* ptr);                                           // freestanding
 
   // \ref{obj.lifetime}, explicit lifetime management
+  template<class T>
+    constexpr void start_lifetime(T& r) noexcept;                                   // freestanding
   template<class T>
     T* start_lifetime_as(void* p) noexcept;                                         // freestanding
   template<class T>
@@ -1036,6 +1038,31 @@
 
 \rSec2[obj.lifetime]{Explicit lifetime management}
 
+\indexlibraryglobal{start_lifetime}%
+\begin{itemdecl}
+template<class T>
+  constexpr void start_lifetime(T& r) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\mandates
+\tcode{T} is a complete type and
+an implicit-lifetime\iref{basic.types} aggregate\iref{dcl.init.aggr}.
+
+\pnum
+\effects
+If the object referenced by \tcode{r}
+is already within its lifetime\iref{basic.life},
+no effects.
+Otherwise, begins the lifetime of the object referenced by \tcode{r}.
+\begin{note}
+No initialization is performed and no subobject has its lifetime started.
+If \tcode{r} denotes a member of a union $U$,
+it becomes the active member of $U$\iref{class.union}.
+\end{note}
+\end{itemdescr}
+
 \indexlibraryglobal{start_lifetime_as}%
 \begin{itemdecl}
 template<class T>

source/preprocessor.tex

@@ -2451,7 +2451,7 @@
 \defnxname{cpp_template_parameters}               & \tcode{202502L} \\ \rowsep
 \defnxname{cpp_template_template_args}            & \tcode{201611L} \\ \rowsep
 \defnxname{cpp_threadsafe_static_init}            & \tcode{200806L} \\ \rowsep
-\defnxname{cpp_trivial_union}                     & \tcode{202502L} \\ \rowsep
+\defnxname{cpp_trivial_union}                     & \tcode{202603L} \\ \rowsep
 \defnxname{cpp_unicode_characters}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_unicode_literals}                  & \tcode{200710L} \\ \rowsep
 \defnxname{cpp_user_defined_literals}             & \tcode{200809L} \\ \rowsep

source/support.tex

@@ -844,6 +844,7 @@
 #define @\defnlibxname{cpp_lib_ssize}@                             201902L // freestanding, also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_sstream_from_string_view}@          202306L // also in \libheader{sstream}
 #define @\defnlibxname{cpp_lib_stacktrace}@                        202011L // also in \libheader{stacktrace}
+#define @\defnlibxname{cpp_lib_start_lifetime}@                    202603L // freestanding, also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_start_lifetime_as}@                 202207L // freestanding, also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_starts_ends_with}@                  201711L // also in \libheader{string}, \libheader{string_view}
 #define @\defnlibxname{cpp_lib_stdatomic_h}@                       202011L // also in \libheader{stdatomic.h}

source/templates.tex

@@ -2574,7 +2574,9 @@
 they are of reference type and they refer to the same object or function, or
 
 \item
-they are of array type and their corresponding elements are template-argument-equivalent,
+they are of array type and their corresponding elements are either
+both within their lifetimes and template-argument-equivalent or
+both not within their lifetimes,
 \begin{footnote}
 An array as a \grammarterm{template-parameter} decays to a pointer.
 \end{footnote}