Merge 2025-06 CWG Motion 6

P3293R3 Splicing a base class subobject

Author: tkoeppe

source/expressions.tex

@@ -4219,7 +4219,10 @@
 
 \pnum
 If \tcode{E2} is a \grammarterm{splice-expression},
-then \tcode{E2} shall designate a member of the type of \tcode{E1}.
+then let \tcode{T1} be the type of \tcode{E1}.
+\tcode{E2} shall designate either
+a member of the \tcode{T1} or
+a direct base class relationship $(\tcode{T1}, \tcode{B})$.
 
 \pnum
 If \tcode{E2} designates a bit-field, \tcode{E1.E2} is a bit-field. The
@@ -4249,7 +4252,7 @@
 \item Otherwise, if \tcode{E2} designates a non-static data member and the type of
 \tcode{E1} is ``\cvqual{cq1 vq1} \tcode{X}'', and the type of \tcode{E2}
 is ``\cvqual{cq2 vq2} \tcode{T}'', the expression designates the corresponding
-member subobject of the object designated by the first expression. If \tcode{E1}
+member subobject of the object designated by \tcode{E1}. If \tcode{E1}
 is an lvalue, then \tcode{E1.E2} is an lvalue;
 otherwise \tcode{E1.E2} is an xvalue.
 Let the notation \cvqual{vq12} stand for the ``union'' of
@@ -4292,6 +4295,40 @@
 is \tcode{T}, the expression \tcode{E1.E2} is a prvalue of type \tcode{T}
 whose value is the value of the enumerator.
 
+\item If \tcode{E2} designates a direct base class relationship $(D, B)$
+and the type of \tcode{E1} is \cv{} \tcode{T},
+the expression designates the direct base class subobject of type $B$
+of the object designated by \tcode{E1}.
+If \tcode{E1} is an lvalue,
+then \tcode{E1.E2} is an lvalue;
+otherwise, \tcode{E1.E2} is an xvalue.
+The type of \tcode{E1.E2} is ``\cv{} \tcode{$B$}''.
+\begin{note}
+This can only occur in an expression of the form \tcode{e1.[:e2:]}.
+\end{note}
+\begin{example}
+\begin{codeblock}
+struct B {
+  int b;
+};
+struct C : B {
+  int get() const { return b; }
+};
+struct D : B, C { };
+
+constexpr int f() {
+  D d = {1, {}};
+
+  // \tcode{b} unambiguously refers to the direct base class of type \tcode{B},
+  // not the indirect base class of type \tcode{B}
+  B& b = d.[: std::meta::bases_of(^^D, std::meta::access_context::current())[0] :];
+  b.b += 10;
+  return 10 * b.b + d.get();
+}
+static_assert(f() == 110);
+\end{codeblock}
+\end{example}
+
 \item Otherwise, the program is ill-formed.
 \end{itemize}
 

source/meta.tex

@@ -2706,12 +2706,14 @@
   consteval bool is_accessible(info r, access_context ctx);
   consteval bool has_inaccessible_nonstatic_data_members(info r, access_context ctx);
   consteval bool has_inaccessible_bases(info r, access_context ctx);
+  consteval bool has_inaccessible_subobjects(info r, access_context ctx);
 
   // \ref{meta.reflection.member.queries}, reflection member queries
   consteval vector<info> members_of(info r, access_context ctx);
   consteval vector<info> bases_of(info type, access_context ctx);
   consteval vector<info> static_data_members_of(info type, access_context ctx);
   consteval vector<info> nonstatic_data_members_of(info type, access_context ctx);
+  consteval vector<info> subobjects_of(info type, access_context ctx);
   consteval vector<info> enumerators_of(info type_enum);
 
   // \ref{meta.reflection.layout}, reflection layout queries
@@ -4589,6 +4591,20 @@
 Otherwise, \tcode{false}.
 \end{itemdescr}
 
+\indexlibraryglobal{has_inaccessible_subobjects}%
+\begin{itemdecl}
+consteval bool has_inaccessible_subobjects(info r, access_context ctx);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+return has_inaccessible_bases(r, ctx) || has_inaccessible_nonstatic_data_members(r, ctx);
+\end{codeblock}
+\end{itemdescr}
+
 \rSec2[meta.reflection.member.queries]{Reflection member queries}
 
 \indexlibraryglobal{members_of}%
@@ -4795,6 +4811,24 @@
 preserving their order.
 \end{itemdescr}
 
+\indexlibraryglobal{subobjects_of}%
+\begin{itemdecl}
+consteval vector<info> subobjects_of(info type, access_context ctx);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\constantwhen
+\tcode{dealias(type)} represents a class type
+that is complete from some point in the evaluation context.
+
+\pnum
+\returns
+A \tcode{vector} containing each element of \tcode{bases_of(type, ctx)}
+followed by each element of \tcode{non\-static_data_mem\-bers_of(\brk{}type,\brk{} ctx)},
+preserving their order.
+\end{itemdescr}
+
 \indexlibraryglobal{enumerators_of}%
 \begin{itemdecl}
 consteval vector<info> enumerators_of(info type_enum);