(arrow) branch main updated: GH-48912: [R] Configure C++20 in conda R on continuous benchmarking (#48974)

[thisisnic]

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The following commit(s) were added to refs/heads/main by this push:
     new de06a522ff GH-48912: [R] Configure C++20 in conda R on continuous 
benchmarking (#48974)
de06a522ff is described below

commit de06a522ffdd32f7d9fd942fe8d1e3305139d780
Author: Nic Crane <thisis...@gmail.com>
AuthorDate: Tue Jan 27 08:37:34 2026 -0500

    GH-48912: [R] Configure C++20 in conda R on continuous benchmarking (#48974)
    
    ### Rationale for this change
    
    Benchmark failing since C++20 upgrade due to lack of C++20 configuration
    
    ### What changes are included in this PR?
    
    Changes entirely from :robot: (Claude) with discussion from me regarding 
optimal approach.
    
    Description as follows:
    
    > conda-forge's R package doesn't have CXX20 configured in Makeconf, even 
though the compiler (gcc 14.3.0) supports C++20. This causes Arrow R package 
installation to fail with "a C++20 compiler is required" because `R CMD config 
CXX20` returns empty.
    >
    > This PR adds CXX20 configuration to R's Makeconf before building the 
Arrow R package in the benchmark hooks, if not already present.
    
    ### Are these changes tested?
    
    I got :robot:  to try it locally in a container but I'm not convinced we'll 
know for sure til we try it out properly.
    
    >  Tested in Docker container with Amazon Linux 2023 + conda-forge R - 
confirmed `R CMD config CXX20` returns empty before patch and `g++` after patch.
    >
    > The only thing we didn't test end-to-end was actually building Arrow R, 
but that would have taken much longer and the configure check (R CMD config 
CXX20 returning non-empty) is exactly what Arrow's configure script tests 
before proceeding.
    
    ### Are there any user-facing changes?
    
    Nope
    * GitHub Issue: #48912
    
    Authored-by: Nic Crane <thisis...@gmail.com>
    Signed-off-by: Nic Crane <thisis...@gmail.com>
---
 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 60a482adfc..5cf75a5c73 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 >> $(R RHOME)/etc/Rprofile.site
+
+  # Ensure CXX20 is configured in R's Makeconf.
+  # conda-forge's R may have empty CXX20 entries even though the compiler 
supports it.
+  # Arrow requires C++20, so we need to add these settings if missing.
+  MAKECONF="$(R RHOME)/etc/Makeconf"
+  if [ -z "$(R CMD config CXX20)" ]; then
+    echo "*** CXX20 not configured in R, adding it to Makeconf"
+    cat >> "$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 .;)
 }