From 98ad67455a61908090cd8b2b4d2b8e7d418538a2 Mon Sep 17 00:00:00 2001
From: Matthias Kretz <m.kretz@gsi.de>
Date: Sat, 28 Mar 2026 17:36:13 +0000
Subject: [PATCH] P3978R3 constant_wrapper should unwrap on call and subscript

---
 source/meta.tex    | 73 +++++++++++++++++++++++++++++++++++++++-------
 source/support.tex |  2 +-
 2 files changed, 64 insertions(+), 11 deletions(-)

diff --git a/source/meta.tex b/source/meta.tex
index 48510a4e39..bf6c2c461c 100644
--- a/source/meta.tex
+++ b/source/meta.tex
@@ -797,16 +797,6 @@
       friend constexpr auto operator->*(L, R) noexcept -> constant_wrapper<L::value->*(R::value)>
         { return {}; }
 
-    // call and index
-    template<@\exposconcept{constexpr-param}@ T, @\exposconcept{constexpr-param}@... Args>
-      constexpr auto operator()(this T, Args...) noexcept
-        requires requires { constant_wrapper<T::value(Args::value...)>(); }
-          { return constant_wrapper<T::value(Args::value...)>{}; }
-    template<@\exposconcept{constexpr-param}@ T, @\exposconcept{constexpr-param}@... Args>
-      constexpr auto operator[](this T, Args...) noexcept
-        -> constant_wrapper<(T::value[Args::value...])>
-          { return {}; }
-
     // pseudo-mutators
     template<@\exposconcept{constexpr-param}@ T>
       constexpr auto operator++(this T) noexcept
@@ -864,6 +854,11 @@
         -> constant_wrapper<X = R::value> { return {}; }
 
     constexpr operator decltype(auto)() const noexcept { return value; }
+
+    template<class... Args>
+      static constexpr decltype(auto) operator()(Args&&... args) noexcept(@\seebelow@);
+    template<class... Args>
+      static constexpr decltype(auto) operator[](Args&&... args) noexcept(@\seebelow@);
   };
 }
 \end{codeblock}
@@ -926,6 +921,64 @@
 Initialize elements of \exposid{data} with corresponding elements of \tcode{arr}.
 \end{itemdescr}
 
+\indexlibrarymember{operator()}{constant_wrapper}%
+\begin{itemdecl}
+template<class... Args>
+  static constexpr decltype(auto) operator()(Args&&... args) noexcept(@\seebelow@);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+Let \tcode{\placeholder{call-expr}} be
+\tcode{constant_wrapper<\placeholder{INVOKE}(value,
+remove_cvref_t<Args>::value...)>\{\}} if all types in
+\tcode{remove_cvref_t<Args>...} satisfy \exposconcept{constexpr-param} and
+\tcode{constant_wrapper<\placeholder{INVOKE}(val\-ue,
+remove_cvref_t<Args>::value...)>} is a valid type, otherwise let
+\tcode{\placeholder{call-expr}} be \tcode{\placeholder{INVOKE}(value,
+std::forward<Args>(args)...)}.
+
+\pnum
+\constraints
+\tcode{\placeholder{call-expr}} is a valid expression.
+
+\pnum
+\effects
+Equivalent to: \tcode{return \placeholder{call-expr};}
+
+\pnum
+\remarks
+The exception specification is equivalent to \tcode{noexcept(\placeholder{call-expr})}.
+\end{itemdescr}
+
+\indexlibrarymember{operator[]}{constant_wrapper}%
+\begin{itemdecl}
+template<class... Args>
+  static constexpr decltype(auto) operator[](Args&&... args) noexcept(@\seebelow@);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+Let \tcode{\placeholder{subscr-expr}} be
+\tcode{constant_wrapper<value[remove_cvref_t<Args>::value...]>\{\}} if all
+types in \tcode{remove_cvref_t<Args>...} satisfy \exposconcept{constexpr-param}
+and \tcode{constant_wrapper<value[remove_cv\-ref_t<Args>::value...]>} is a valid
+type, otherwise let \tcode{\placeholder{subscr-expr}} be
+\tcode{value[std::forward<Args\brk{}>(args)...]}.
+
+\pnum
+\constraints
+\tcode{\placeholder{subscr-expr}} is a valid expression.
+
+\pnum
+\effects
+Equivalent to: \tcode{return \placeholder{subscr-expr};}
+
+\pnum
+\remarks
+The exception specification is equivalent to \tcode{noexcept(\placeholder{subscr-expr})}.
+\end{itemdescr}
+
 \rSec2[meta.unary]{Unary type traits}
 
 \rSec3[meta.unary.general]{General}
diff --git a/source/support.tex b/source/support.tex
index f4f632944b..a2639f9af2 100644
--- a/source/support.tex
+++ b/source/support.tex
@@ -620,7 +620,7 @@
 #define @\defnlibxname{cpp_lib_complex_udls}@                      201309L // also in \libheader{complex}
 #define @\defnlibxname{cpp_lib_concepts}@                          202207L
   // freestanding, also in \libheader{concepts}, \libheader{compare}
-#define @\defnlibxname{cpp_lib_constant_wrapper}@                  202506L // freestanding, also in \libheader{type_traits}
+#define @\defnlibxname{cpp_lib_constant_wrapper}@                  202603L // freestanding, also in \libheader{utility}
 #define @\defnlibxname{cpp_lib_constexpr_algorithms}@              202306L // also in \libheader{algorithm}, \libheader{utility}
 #define @\defnlibxname{cpp_lib_constexpr_atomic}@                  202411L // also in \libheader{atomic}
 #define @\defnlibxname{cpp_lib_constexpr_bitset}@                  202207L // also in \libheader{bitset}