Merge 2019-07 LWG Motion 31

P0466R5 Layout-compatibility and Pointer-interconvertibility Traits

Fixes #3035.

Author: zygoloid

source/utilities.tex

@@ -16275,7 +16275,9 @@
 \rSec2[meta.type.synop]{Header \tcode{<type_traits>} synopsis}
 
 \indexhdr{type_traits}%
+\indexlibrary{\idxcode{is_layout_compatible_v}}%
 \indexlibrary{\idxcode{is_nothrow_convertible_v}}%
+\indexlibrary{\idxcode{is_pointer_interconvertible_base_of_v}}%
 \indexlibrary{\idxcode{type_identity_t}}%
 \indexlibrary{\idxcode{common_reference_t}}%
 % FIXME: Many index entries missing.
@@ -16385,6 +16387,8 @@
   template<class Base, class Derived> struct is_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;
+  template<class Base, class Derived> struct is_pointer_interconvertible_base_of;
 
   template<class Fn, class... ArgTypes> struct is_invocable;
   template<class R, class Fn, class... ArgTypes> struct is_invocable_r;
@@ -16666,6 +16670,11 @@
     inline constexpr bool is_convertible_v = is_convertible<From, To>::value;
   template<class From, class To>
     inline constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<From, To>::value;
+  template<class T, class U>
+    inline constexpr bool is_layout_compatible_v = is_layout_compatible<T, U>::value;
+  template<class Base, class Derived>
+    inline constexpr bool is_pointer_interconvertible_base_of_v
+      = is_pointer_interconvertible_base_of<Base, Derived>::value;
   template<class Fn, class... ArgTypes>
     inline constexpr bool is_invocable_v = is_invocable<Fn, ArgTypes...>::value;
   template<class R, class Fn, class... ArgTypes>
@@ -16684,6 +16693,12 @@
   template<class B>
     inline constexpr bool negation_v = negation<B>::value;
 
+  // \ref{meta.member}, member relationships
+  template<class S, class M>
+    constexpr bool is_pointer_interconvertible_with_class(M S::*m) noexcept;
+  template<class S1, class S2, class M1, class M2>
+    constexpr bool is_corresponding_member(M1 S1::*m1, M2 S2::*m2) noexcept;
+
   // \ref{meta.const.eval}, constant evaluation context
   constexpr bool is_constant_evaluated() noexcept;
 }
@@ -17498,6 +17513,29 @@
  types, arrays of unknown
  bound, or \cv{}~\tcode{void} types.                \\ \rowsep
 
+\indexlibrary{\idxcode{is_layout_compatible}}%
+\tcode{template<class T, class U>}\br
+ \tcode{struct is_layout_compatible;}                 &
+ \tcode{T} and \tcode{U} are layout-compatible\iref{basic.types}    &
+ \tcode{T} and \tcode{U} shall be complete types,
+ \cv{}~\tcode{void},
+ or arrays of unknown bound.                \\ \rowsep
+
+\indexlibrary{\idxcode{is_pointer_interconvertible_base_of}}%
+\tcode{template<class Base, class Derived>}\br
+ \tcode{struct is_pointer_interconvertible_base_of;}                 &
+ \tcode{Derived} is unambiguously derived from \tcode{Base}
+ without regard to cv-qualifiers,
+ and each object of type \tcode{Derived}
+ is pointer-interconvertible\iref{basic.compound} with
+ its \tcode{Base} subobject,
+ or \tcode{Base} and \tcode{Derived} are not unions
+ and name the same class type
+ 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,
+ \tcode{Derived} shall be a complete type.  \\ \rowsep
+
 \indexlibrary{\idxcode{is_invocable}}%
 \tcode{template<class Fn, class... ArgTypes>}\br
  \tcode{struct is_invocable;}                      &
@@ -18315,6 +18353,75 @@
 is a \oldconcept{UnaryTypeTrait} with a base characteristic of \tcode{bool_constant<!bool(B::\brk{}value)>}.
 \end{itemdescr}
 
+\rSec2[meta.member]{Member relationships}
+
+\indexlibrary{\idxcode{is_pointer_interconvertible_with_class}}
+\begin{itemdecl}
+template<class S, class M>
+  constexpr bool is_pointer_interconvertible_with_class(M S::*m) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\mandates \tcode{S} is a complete type.
+
+\pnum
+\returns \tcode{true} if and only if
+ \tcode{S} is a standard-layout type,
+ \tcode{M} is an object type,
+ \tcode{m} is not null,
+ and each object \tcode{s} of type \tcode{S}
+ is pointer-interconvertible\iref{basic.compound}
+ with its subobject \tcode{s.*m}.
+\end{itemdescr}
+
+\indexlibrary{\idxcode{is_corresponding_member}}
+\begin{itemdecl}
+template<class S1, class S2, class M1, class M2>
+  constexpr bool is_corresponding_member(M1 S1::*m1, M2 S2::*m2) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\mandates \tcode{S1} and \tcode{S2} are complete types.
+
+\pnum
+\returns \tcode{true} if and only if
+ \tcode{S1} and \tcode{S2} are standard-layout types,
+ \tcode{M1} and \tcode{M2} are object types,
+ \tcode{m1} and \tcode{m2} are not null,
+ and \tcode{m1} and \tcode{m2} point to corresponding members of
+ the common initial sequence\iref{class.mem} of \tcode{S1} and \tcode{S2}.
+\end{itemdescr}
+
+\pnum
+\begin{note}
+The type of a pointer-to-member expression \tcode{\&C::b}
+is not always a pointer to member of \tcode{C},
+leading to potentially surprising results
+when using these functions in conjunction with inheritance.
+\begin{example}
+\begin{codeblock}
+struct A { int a; };                    // a standard-layout class
+struct B { int b; };                    // a standard-layout class
+struct C: public A, public B { };       // not a standard-layout class
+
+static_assert( is_pointer_interconvertible_with_class( &C::b ) );
+  // Succeeds because, despite its appearance, \tcode{\&C::b} has type
+  // ``pointer to member of \tcode{B} of type \tcode{int}''.
+static_assert( is_pointer_interconvertible_with_class<C>( &C::b ) );
+  // Forces the use of class \tcode{C}, and fails.
+
+static_assert( is_corresponding_member( &C::a, &C::b ) );
+  // Succeeds because, despite its appearance, \tcode{\&C::a} and \tcode{\&C::b} have types
+  // ``pointer to member of \tcode{A} of type \tcode{int}'' and
+  // ``pointer to member of \tcode{B} of type \tcode{int}'', respectively.
+static_assert( is_corresponding_member<C, C>( &C::a, &C::b ) );
+  // Forces the use of class \tcode{C}, and fails.
+\end{codeblock}
+\end{example}
+\end{note}
+
 \rSec2[meta.const.eval]{Constant evaluation context}
 \begin{itemdecl}
 constexpr bool is_constant_evaluated() noexcept;