From fbe2d82ef77733700ce96d01e5918f44f1954938 Mon Sep 17 00:00:00 2001
From: Nic Crane <thisisnic@gmail.com>
Date: Sat, 24 Jan 2026 11:18:43 -0500
Subject: [PATCH 1/2] Add cpp20 config

---
 dev/conbench_envs/hooks.sh | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/dev/conbench_envs/hooks.sh b/dev/conbench_envs/hooks.sh
index 60a482adfcf..dc81ceeaf23 100755
--- a/dev/conbench_envs/hooks.sh
+++ b/dev/conbench_envs/hooks.sh
@@ -59,6 +59,25 @@ build_arrow_python() {
 
 build_arrow_r() {
   cat ci/etc/rprofile >> $(R RHOME)/etc/Rprofile.site
+
+  # Ensure CXX20 is configured in R's Makeconf.
+  # conda-forge's R may not have CXX20 set even though the compiler supports it.
+  # Arrow requires C++20, so we need to add these settings if missing.
+  MAKECONF="$(R RHOME)/etc/Makeconf"
+  if ! grep -q "^CXX20 " "$MAKECONF"; then
+    echo "*** CXX20 not configured in R, adding it to Makeconf"
+    cat >> "$MAKECONF" << 'EOF'
+
+# Added for Arrow C++20 support
+CXX20 = g++
+CXX20FLAGS = -g -O2 $(LTO)
+CXX20PICFLAGS = -fpic
+CXX20STD = -std=gnu++20
+SHLIB_CXX20LD = $(CXX20) $(CXX20STD)
+SHLIB_CXX20LDFLAGS = -shared
+EOF
+  fi
+
   ci/scripts/r_deps.sh $(pwd) $(pwd)
   (cd r; R CMD INSTALL .;)
 }

From f164e4dfe6241749f5a080ccb673cea7fe9d0ece Mon Sep 17 00:00:00 2001
From: Nic Crane <thisisnic@gmail.com>
Date: Sat, 24 Jan 2026 11:40:24 -0500
Subject: [PATCH 2/2] Check more thoroughly for config not existence

---
 dev/conbench_envs/hooks.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/dev/conbench_envs/hooks.sh b/dev/conbench_envs/hooks.sh
index dc81ceeaf23..5cf75a5c734 100755
--- a/dev/conbench_envs/hooks.sh
+++ b/dev/conbench_envs/hooks.sh
@@ -61,10 +61,10 @@ build_arrow_r() {
   cat ci/etc/rprofile >> $(R RHOME)/etc/Rprofile.site
 
   # Ensure CXX20 is configured in R's Makeconf.
-  # conda-forge's R may not have CXX20 set even though the compiler supports it.
+  # conda-forge's R may have empty CXX20 entries even though the compiler supports it.
   # Arrow requires C++20, so we need to add these settings if missing.
   MAKECONF="$(R RHOME)/etc/Makeconf"
-  if ! grep -q "^CXX20 " "$MAKECONF"; then
+  if [ -z "$(R CMD config CXX20)" ]; then
     echo "*** CXX20 not configured in R, adding it to Makeconf"
     cat >> "$MAKECONF" << 'EOF'