P2445R1 std::forward_like

source/support.tex

@@ -625,6 +625,7 @@
 #define @\defnlibxname{cpp_lib_find_last}@                         202207L // also in \libheader{algorithm}
 #define @\defnlibxname{cpp_lib_flat_map}@                          202207L // also in \libheader{flat_map}
 #define @\defnlibxname{cpp_lib_format}@                            202207L // also in \libheader{format}
+#define @\defnlibxname{cpp_lib_forward_like}@                      202207L // also in \libheader{utility}
 #define @\defnlibxname{cpp_lib_gcd_lcm}@                           201606L // also in \libheader{numeric}
 #define @\defnlibxname{cpp_lib_generic_associative_lookup}@        201304L // also in \libheader{map}, \libheader{set}
 #define @\defnlibxname{cpp_lib_generic_unordered_lookup}@          201811L

source/utilities.tex

@@ -55,6 +55,8 @@
     constexpr T&& forward(remove_reference_t<T>& t) noexcept;
   template<class T>
     constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
+  template<class T, class U>
+    [[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> @\seebelow@;
   template<class T>
     constexpr remove_reference_t<T>&& move(T&&) noexcept;
   template<class T>
@@ -358,6 +360,57 @@
 \end{example}
 \end{itemdescr}
 
+\indexlibraryglobal{forward_like}%
+\begin{itemdecl}
+template<class T, class U>
+  [[nodiscard]] constexpr auto forward_like(U&& x) noexcept -> @\seebelow@;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\begin{itemize}
+\item
+Let \tcode{\exposid{COPY_CONST}(A, B)} be \tcode{const B}
+if \tcode{A} is a const type, otherwise \tcode{B}.
+\item
+Let \tcode{\exposid{OVERRIDE_REF}(A, B)} be \tcode{remove_reference_t<B>\&\&}
+if \tcode{A} is an rvalue reference type, otherwise \tcode{B\&}.
+\item
+Let \tcode{V} be
+\begin{codeblock}
+@\exposid{OVERRIDE_REF}@(T&&, @\exposid{COPY_CONST}@(remove_reference_t<T>, remove_reference_t<U>))
+\end{codeblock}
+\end{itemize}
+
+\pnum
+\returns
+\tcode{static_cast<V>(x)}.
+
+\pnum
+\remarks
+The return type is \tcode{V}.
+
+\pnum
+\begin{example}
+\begin{codeblock}
+struct accessor {
+  vector<string>* container;
+  decltype(auto) operator[](this auto&& self, size_t i) {
+    return std::forward_like<decltype(self)>((*container)[i]);
+  }
+};
+void g() {
+  vector v{"a"s, "b"s};
+  accessor a{&v};
+  string& x = a[0];                             // OK, binds to lvalue reference
+  string&& y = std::move(a)[0];                 // OK, is rvalue reference
+  string const&& z = std::move(as_const(a))[1]; // OK, is \tcode{const\&\&}
+  string& w = as_const(a)[1];                   // error: will not bind to non-const
+}
+\end{codeblock}
+\end{example}
+\end{itemdescr}
+
 \indexlibrary{\idxcode{move}!function}%
 \indextext{\idxcode{move}}%
 \begin{itemdecl}