From 97844be578860cdf853b4c560cb7669670d7b12b Mon Sep 17 00:00:00 2001
From: Henry Schreiner <henryfs@princeton.edu>
Date: Wed, 22 Apr 2026 17:15:56 -0400
Subject: [PATCH] fix: avoid dangling-reference in sample tuple conversion for
 C++23 compatibility

Under C++23 P2255R2 (Type Traits To Detect References Binding To
Temporaries), std::tuple's converting constructor is deleted when a
reference element would bind to a temporary. This broke code that
converts tuple<int> to tuple<const double&>, since the int is
first converted to a temporary double.

Fix: strip cv-qualifiers and reference from the deduced sample
argument types in accumulator_traits. This makes
accumulator_traits<mean<double>>::args == tuple<double> instead of
tuple<const double&>, so the compile-time convertibility check
is_convertible<tuple<int>, tuple<double>> succeeds.

Runtime behavior is unchanged; accumulators still receive arguments
through their const double& parameter.

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>
Assisted-by: OpenCode:Kimi-K2.6
---
 .../boost/histogram/detail/accumulator_traits.hpp | 15 ++++++++++-----
 test/accumulators_collector_test.cpp              |  2 +-
 test/accumulators_mean_test.cpp                   |  2 +-
 test/detail_accumulator_traits_test.cpp           |  2 +-
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/include/boost/histogram/detail/accumulator_traits.hpp b/include/boost/histogram/detail/accumulator_traits.hpp
index e50503bf..76269ea8 100644
--- a/include/boost/histogram/detail/accumulator_traits.hpp
+++ b/include/boost/histogram/detail/accumulator_traits.hpp
@@ -31,24 +31,29 @@ struct accumulator_traits_holder {
 
 // member function pointer with weight_type as first argument is better match
 template <class R, class T, class U, class... Ts>
-accumulator_traits_holder<true, Ts...> accumulator_traits_impl_call_op(
+accumulator_traits_holder<true, std::remove_cv_t<std::remove_reference_t<Ts>>...>
+accumulator_traits_impl_call_op(
     R (T::*)(boost::histogram::weight_type<U>, Ts...));
 
 template <class R, class T, class U, class... Ts>
-accumulator_traits_holder<true, Ts...> accumulator_traits_impl_call_op(
+accumulator_traits_holder<true, std::remove_cv_t<std::remove_reference_t<Ts>>...>
+accumulator_traits_impl_call_op(
     R (T::*)(boost::histogram::weight_type<U>&, Ts...));
 
 template <class R, class T, class U, class... Ts>
-accumulator_traits_holder<true, Ts...> accumulator_traits_impl_call_op(
+accumulator_traits_holder<true, std::remove_cv_t<std::remove_reference_t<Ts>>...>
+accumulator_traits_impl_call_op(
     R (T::*)(boost::histogram::weight_type<U>&&, Ts...));
 
 template <class R, class T, class U, class... Ts>
-accumulator_traits_holder<true, Ts...> accumulator_traits_impl_call_op(
+accumulator_traits_holder<true, std::remove_cv_t<std::remove_reference_t<Ts>>...>
+accumulator_traits_impl_call_op(
     R (T::*)(const boost::histogram::weight_type<U>&, Ts...));
 
 // member function pointer only considered if all specializations above fail
 template <class R, class T, class... Ts>
-accumulator_traits_holder<false, Ts...> accumulator_traits_impl_call_op(R (T::*)(Ts...));
+accumulator_traits_holder<false, std::remove_cv_t<std::remove_reference_t<Ts>>...>
+accumulator_traits_impl_call_op(R (T::*)(Ts...));
 
 template <class T>
 auto accumulator_traits_impl(T&, priority<2>)
diff --git a/test/accumulators_collector_test.cpp b/test/accumulators_collector_test.cpp
index 3207614a..2f9f841c 100644
--- a/test/accumulators_collector_test.cpp
+++ b/test/accumulators_collector_test.cpp
@@ -30,7 +30,7 @@ bool operator==(const span<T1>& a, const std::vector<T2>& b) {
 int main() {
   using traits = detail::accumulator_traits<accumulators::collector<>>;
   static_assert(!traits::weight_support, "");
-  static_assert(std::is_same<traits::args, std::tuple<const double&>>::value, "");
+  static_assert(std::is_same<traits::args, std::tuple<double>>::value, "");
 
   {
     accumulators::collector<std::vector<int>> acc;
diff --git a/test/accumulators_mean_test.cpp b/test/accumulators_mean_test.cpp
index bf737803..f8a4e8ed 100644
--- a/test/accumulators_mean_test.cpp
+++ b/test/accumulators_mean_test.cpp
@@ -24,7 +24,7 @@ int main() {
 
   using traits = detail::accumulator_traits<m_t>;
   static_assert(traits::weight_support, "");
-  static_assert(std::is_same<traits::args, std::tuple<const double&>>::value, "");
+  static_assert(std::is_same<traits::args, std::tuple<double>>::value, "");
 
   // basic interface, string conversion
   {
diff --git a/test/detail_accumulator_traits_test.cpp b/test/detail_accumulator_traits_test.cpp
index 6a36ba6e..979b0ce0 100644
--- a/test/detail_accumulator_traits_test.cpp
+++ b/test/detail_accumulator_traits_test.cpp
@@ -72,7 +72,7 @@ int main() {
   };
 
   BOOST_TEST(dtl::accumulator_traits<A7>::weight_support);
-  BOOST_TEST_TRAIT_SAME(typename dtl::accumulator_traits<A7>::args, std::tuple<int&&>);
+  BOOST_TEST_TRAIT_SAME(typename dtl::accumulator_traits<A7>::args, std::tuple<int>);
 
   struct B1 {
     int operator+=(int);