[LWG motion 6 2024-06] P2985R0 A type trait for detecting virtual base classes

source/meta.tex

@@ -262,6 +262,7 @@
   // \ref{meta.rel}, type relations
   template<class T, class U> struct is_same;
   template<class Base, class Derived> struct is_base_of;
+  template<class Base, class Derived> struct is_virtual_base_of;
   template<class From, class To> struct is_convertible;
   template<class From, class To> struct is_nothrow_convertible;
   template<class T, class U> struct is_layout_compatible;
@@ -548,6 +549,8 @@
     constexpr bool @\libglobal{is_same_v}@ = is_same<T, U>::value;
   template<class Base, class Derived>
     constexpr bool @\libglobal{is_base_of_v}@ = is_base_of<Base, Derived>::value;
+  template<class Base, class Derived>
+    constexpr bool @\libglobal{is_virtual_base_of_v}@ = is_virtual_base_of<Base, Derived>::value;
   template<class From, class To>
     constexpr bool @\libglobal{is_convertible_v}@ = is_convertible<From, To>::value;
   template<class From, class To>
@@ -1463,15 +1466,31 @@
  without regard to cv-qualifiers            &
  If \tcode{Base} and
  \tcode{Derived} are non-union class types and are
-not (possibly cv-qualified versions of) the same type,
+ not (possibly cv-qualified versions of) the same type,
  \tcode{Derived} shall be a complete
  type.
  \begin{tailnote}
-Base classes that are private, protected, or ambiguous
+ Base classes that are private, protected, or ambiguous
  are, nonetheless, base classes.
 \end{tailnote}
 \\ \rowsep
 
+\indexlibraryglobal{is_virtual_base_of}%
+\tcode{template<class Base, class Derived>}\br
+ \tcode{struct is_virtual_base_of;}                 &
+ \tcode{Base} is a virtual base class of \tcode{Derived}\iref{class.mi}
+ without regard to cv-qualifiers.            &
+ If \tcode{Base} and
+ \tcode{Derived} are non-union class types,
+ \tcode{Derived} shall be a complete type.
+ \begin{note}
+ Virtual base classes that are private, protected, or ambiguous
+ are, nonetheless, virtual base classes.
+ \end{note}
+ \begin{tailnote}
+ A class is never a virtual base class of itself.
+ \end{tailnote}                                     \\ \rowsep
+
 \indexlibraryglobal{is_convertible}%
 \tcode{template<class From, class To>}\br
  \tcode{struct is_convertible;}             &
@@ -1486,7 +1505,7 @@
  the conversion, as defined by \tcode{is_convertible},
  is known not to throw any exceptions\iref{expr.unary.noexcept} &
  \tcode{From} and \tcode{To} shall be complete types,
- \cv{}~\keyword{void}, or arrays of unknown bound. \\ \rowsep
+ \cv{}~\keyword{void}, or arrays of unknown bound.  \\ \rowsep
 
 \indexlibraryglobal{is_layout_compatible}%
 \tcode{template<class T, class U>}\br

source/support.tex

@@ -700,6 +700,7 @@
 #define @\defnlibxname{cpp_lib_is_pointer_interconvertible}@       201907L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_scoped_enum}@                    202011L // freestanding, also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_swappable}@                      201603L // freestanding, also in \libheader{type_traits}
+#define @\defnlibxname{cpp_lib_is_virtual_base_of}@                202406L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_within_lifetime}@                202306L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_jthread}@                           201911L // also in \libheader{stop_token}, \libheader{thread}
 #define @\defnlibxname{cpp_lib_latch}@                             201907L // also in \libheader{latch}