P2136R3 invoke_r

Also fixes LWG2690.

Author: jensmaurer

source/support.tex

@@ -625,6 +625,7 @@
 #define @\defnlibxname{cpp_lib_integral_constant_callable}@        201304L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_interpolate}@                       201902L // also in \libheader{cmath}, \libheader{numeric}
 #define @\defnlibxname{cpp_lib_invoke}@                            201411L // also in \libheader{functional}
+#define @\defnlibxname{cpp_lib_invoke_r}@                          202106L // also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_is_aggregate}@                      201703L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_constant_evaluated}@             201811L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_is_final}@                          201402L // also in \libheader{type_traits}

source/utilities.tex

@@ -13490,6 +13490,10 @@
     constexpr invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
       noexcept(is_nothrow_invocable_v<F, Args...>);
 
+  template<class R, class F, class... Args>
+    constexpr R invoke_r(F&& f, Args&&... args)
+      noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
+
   // \ref{refwrap}, \tcode{reference_wrapper}
   template<class T> class reference_wrapper;
 
@@ -13793,7 +13797,7 @@
 a given standard library function template have the same type
 if the types of their corresponding state entities are the same.
 
-\rSec2[func.invoke]{Function template \tcode{invoke}}
+\rSec2[func.invoke]{\tcode{invoke} functions}
 \indexlibraryglobal{invoke}%
 \indexlibrary{invoke@\tcode{\placeholder{INVOKE}}}%
 \begin{itemdecl}
@@ -13803,11 +13807,32 @@
 \end{itemdecl}
 
 \begin{itemdescr}
+\pnum
+\constraints
+\tcode{is_invocable_v<F, Args...>} is \tcode{true}.
+
 \pnum
 \returns
 \tcode{\placeholdernc{INVOKE}(std::forward<F>(f), std::forward<Args>(args)...)}\iref{func.require}.
 \end{itemdescr}
 
+\indexlibraryglobal{invoke_r}%
+\begin{itemdecl}
+template<class R, class F, class... Args>
+  constexpr R invoke_r(F&& f, Args&&... args)
+    noexcept(is_nothrow_invocable_r_v<R, F, Args...>);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\constraints
+\tcode{is_invocable_r_v<R, F, Args...>} is \tcode{true}.
+
+\pnum
+\returns
+\tcode{\placeholdernc{INVOKE}<R>(std::forward<F>(f), std::forward<Args>(args)...)}\iref{func.require}.
+\end{itemdescr}
+
 \rSec2[refwrap]{Class template \tcode{reference_wrapper}}
 
 \rSec3[refwrap.general]{General}