P3074R7 trivial unions (was std::uninitialized<T>)

Author: jensmaurer

source/classes.tex

@@ -1220,59 +1220,64 @@
 \item any non-static data member with no default member initializer\iref{class.mem} is
 of reference type,
 
-\item any non-variant non-static data member of const-qualified type
+\item \tcode{X} is a non-union class and
+any non-variant non-static data member of const-qualified type
 (or possibly multidimensional array thereof)
 with no \grammarterm{brace-or-equal-initializer}
 is not const-default-constructible\iref{dcl.init},
 
-\item \tcode{X} is a union and all of its variant members are of const-qualified
-type (or possibly multidimensional array thereof),
-
-\item \tcode{X} is a non-union class and all members of any anonymous union member are
-of const-qualified type (or possibly multidimensional array thereof),
-
-\item any potentially constructed subobject, except for a non-static data member
-with a \grammarterm{brace-or-equal-initializer}
-or a variant member of a union where another non-static data member
-has a \grammarterm{brace-or-equal-initializer},
+\item any non-variant potentially constructed subobject, except for a non-static data member
+with a \grammarterm{brace-or-equal-initializer},
 has class type \tcode{M} (or possibly multidimensional array thereof)
 and overload resolution\iref{over.match}
 as applied to find \tcode{M}'s corresponding constructor
-either does not result in a usable candidate\iref{over.match.general}
-or, in the case of a variant member, selects a non-trivial function, or
+does not result in a usable candidate\iref{over.match.general}, or
 
-\item any potentially constructed subobject has
-class type \tcode{M} (or possibly multidimensional array thereof) and
+\item any potentially constructed subobject $S$ has
+class type \tcode{M} (or possibly multidimensional array thereof),
 \tcode{M} has
 a destructor that is deleted or inaccessible from the defaulted default
-constructor.
+constructor, and
+either $S$ is non-variant or $S$ has a default member initializer.
 \end{itemize}
 
 \pnum
-A default constructor is
+A default constructor for a class \tcode{X} is
 \defnx{trivial}{constructor!default!trivial}
 if it is not user-provided and if
 \begin{itemize}
 \item
-its class has no virtual functions\iref{class.virtual} and no virtual base
+\tcode{X} has no virtual functions\iref{class.virtual} and no virtual base
 classes\iref{class.mi}, and
 
-\item no non-static data member of its class has
+\item no non-static data member of \tcode{X} has
 a default member initializer\iref{class.mem}, and
 
 \item
-all the direct base classes of its class have trivial default constructors, and
+all the direct base classes of \tcode{X} have trivial default constructors, and
 
 \item
-for all the non-static data members of its class that are of class
+either \tcode{X} is a union or
+for all the non-variant non-static data members of \tcode{X} that are of class
 type (or array thereof), each such class has a trivial default constructor.
 \end{itemize}
 
 Otherwise, the default constructor is
 \defnx{non-trivial}{constructor!default!non-trivial}.
 
 \pnum
-An implicitly-defined\iref{dcl.fct.def.default} default constructor performs the set of
+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
 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
@@ -2022,25 +2027,40 @@
 A defaulted destructor for a class
   \tcode{X} is defined as deleted if
 \begin{itemize}
-\item any potentially constructed subobject has class type
-  \tcode{M} (or possibly multidimensional array thereof) and
+\item \tcode{X} is a non-union class and
+  any non-variant potentially constructed subobject has class type
+  \tcode{M} (or possibly multidimensional array thereof) where
   \tcode{M} has a destructor that is deleted or
-  is inaccessible from the defaulted destructor or,
-  in the case of a variant member, is non-trivial,
+  is inaccessible from the defaulted destructor,
 
-\item or, for a virtual destructor, lookup of the non-array deallocation
+\item
+  \tcode{X} is a union and
+  \begin{itemize}
+  \item
+    overload resolution to select a constructor to
+    default-initialize an object of type \tcode{X} either fails or
+    selects a constructor that is either deleted or not trivial, or
+  \item
+    \tcode{X} has a variant member \tcode{V} of
+    class type \tcode{M} (or possibly multi-dimensional array thereof)
+    where \tcode{V} has a default member initializer and
+    \tcode{M} has a destructor that is non-trivial, or,
+  \end{itemize}
+
+\item for a virtual destructor, lookup of the non-array deallocation
   function results in an ambiguity or in a function that is deleted or
   inaccessible from the defaulted destructor.
 \end{itemize}
 
 \pnum
-A destructor is trivial if it is not user-provided and if
+A destructor for a class \tcode{X} is trivial if it is not user-provided and if
 \begin{itemize}
 \item the destructor is not virtual,
 
-\item all of the direct base classes of its class have trivial destructors, and
+\item all of the direct base classes of \tcode{X} have trivial destructors, and
 
-\item for all of the non-static data members of its class that are of class
+\item either \tcode{X} is a union or
+for all of the non-variant non-static data members of \tcode{X} that are of class
 type (or array thereof), each such class has a trivial destructor.
 \end{itemize}
 
@@ -3054,12 +3074,12 @@
 If a union contains a non-static data member of
 reference type, the program is ill-formed.
 \begin{note}
-Absent default member initializers\iref{class.mem},
-if any non-static data member of a union has a non-trivial
-default constructor\iref{class.default.ctor},
-copy constructor, move constructor\iref{class.copy.ctor},
-copy assignment operator, move assignment operator\iref{class.copy.assign},
-or destructor\iref{class.dtor}, the corresponding member function
+If any non-static data member of a union has a non-trivial
+copy constructor,
+move constructor\iref{class.copy.ctor},
+copy assignment operator, or
+move assignment operator\iref{class.copy.assign},
+the corresponding member function
 of the union must be user-provided or it will
 be implicitly deleted\iref{dcl.fct.def.delete} for the union.
 \begin{example}
@@ -3072,11 +3092,11 @@
 };
 \end{codeblock}
 Since \tcode{std::string}\iref{string.classes} declares non-trivial versions of all of the special
-member functions, \tcode{U} will have an implicitly deleted default constructor,
-copy/move constructor,
-copy/move assignment operator, and destructor.
-To use \tcode{U}, some or all of these member functions
-must be user-provided.
+member functions, \tcode{U} will have an implicitly deleted
+copy/move constructor and copy/move assignment operator.
+The default constructor and destructor of \tcode{U} are both trivial
+even though \tcode{std::string} has
+a non-trivial default constructor and a non-trivial destructor.
 \end{example}
 \end{note}
 

source/containers.tex

@@ -9534,13 +9534,6 @@
 \tcode{inplace_vector<T, N>::const_iterator}
 meet the constexpr iterator requirements.
 
-\pnum
-For any $\tcode{N} > 0$,
-if \tcode{T} is not trivially copyable or
-\tcode{is_trivially_default_constructible_v<T>} is \tcode{false},
-then no \tcode{inplace_vector<T, N>} member functions
-are usable in constant expressions.
-
 \pnum
 Any member function of \tcode{inplace_vector<T, N>} that
 would cause the size to exceed \tcode{N}

source/preprocessor.tex

@@ -1908,6 +1908,7 @@
 \defnxname{cpp_structured_bindings}               & \tcode{202411L} \\ \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_unicode_characters}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_unicode_literals}                  & \tcode{200710L} \\ \rowsep
 \defnxname{cpp_user_defined_literals}             & \tcode{200809L} \\ \rowsep

source/support.tex

@@ -612,6 +612,7 @@
 #define @\defnlibxname{cpp_lib_constexpr_dynamic_alloc}@           201907L // also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_constexpr_exceptions}@              202411L // also in \libheader{exception}
 #define @\defnlibxname{cpp_lib_constexpr_functional}@              201907L // freestanding, also in \libheader{functional}
+#define @\defnlibxname{cpp_lib_constexpr_inplace_vector}@          202502L // also in \libheader{inplace_vector}
 #define @\defnlibxname{cpp_lib_constexpr_iterator}@                201811L // freestanding, also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_constexpr_memory}@                  202202L // freestanding, also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_constexpr_new}@                     202406L // freestanding, also in \libheader{new}