build: update supported C++ standards to C++23, C++26

Since C++26 was completed [1], we retire C++20 support and
introduce C++26. The tests matrix was regenerated.

Cooking fmt version updated to 12.2.0 to include a bug fix for
std::optional formatting with C++26 [2].

The build environment for testing was updated to cook fmt for C++26,
not just clang.

[1] https://herbsutter.com/2026/03/29/c26-is-done-trip-report-march-2026-iso-c-standards-meeting-london-croydon-uk/

[2] fmtlib/fmt#4760

Closes scylladb#3462

Author: avikivity

.github/workflows/gen-matrix.py

@@ -62,7 +62,7 @@
     [f"clang++-{v}" for v in CLANG_VERSIONS]
     + [f"g++-{v}"   for v in GCC_VERSIONS]
 )
-STANDARDS: list[int] = [20, 23]
+STANDARDS: list[int] = [23, 26]
 MODES: list[str] = ["debug", "release", "sanitize"]
 ARCHS: list[str] = ["x86", "arm"]
 

.github/workflows/install-build-env.sh

@@ -87,18 +87,17 @@ if [[ "$ENABLE_CCACHE" != "false" ]]; then
     ccache_opt=(--compiler-cache=ccache)
 fi
 
-# --cook fmt for clang >= 20: the system libfmt-dev on ubuntu:26.04 is
+# --cook fmt for clang >= 20 or C++26: the system libfmt-dev on ubuntu:26.04 is
 # 10.1.1, and clang-20+ enforces consteval strictly enough to reject
-# fmt/chrono.h's FMT_STRING("{:.{}f}") path. The gcc matrix items
-# work fine with the system fmt. The check is version-guarded rather
+# fmt/chrono.h's FMT_STRING("{:.{}f}") path. C++26 requires a fmt fix
+# for std::optional formatting (https://github.com/fmtlib/fmt/issues/4760)
+# The check is version-guarded rather
 # than blanket-applied so future pre-20 clang versions in the matrix
 # don't pay the build cost.
 cook_args=()
-if [[ "$COMPILER" == clang++-* ]]; then
-    clang_ver="${COMPILER#clang++-}"
-    if [ "$clang_ver" -ge 20 ]; then
-        cook_args=(--cook fmt)
-    fi
+clang_ver="${COMPILER#clang++-}"
+if [[ "$COMPILER" == clang++-* && "$clang_ver" -ge 20 || "$STANDARD" -ge 26 ]] ; then
+    cook_args=(--cook fmt)
 fi
 
 group "configure.py"

.github/workflows/tests.yaml

@@ -22,15 +22,15 @@ jobs:
         # This include block is AUTOGENERATED, do not edit by hand
         # instead edit + re-run .github/workflows/gen-matrix.py
         include:
-          - {compiler: clang++-21, standard: 20, mode: debug, arch: x86}
-          - {compiler: clang++-22, standard: 23, mode: release, arch: x86}
-          - {compiler: g++-15, standard: 23, mode: release, arch: arm}
-          - {compiler: g++-16, standard: 20, mode: release, arch: arm}
-          - {compiler: g++-16, standard: 23, mode: release, arch: x86}
-          - {compiler: g++-15, standard: 20, mode: release, arch: x86}
-          - {compiler: clang++-22, standard: 20, mode: sanitize, arch: arm}
-          - {compiler: clang++-21, standard: 23, mode: sanitize, arch: x86}
-          - {compiler: clang++-21, standard: 23, mode: debug, arch: arm}
+          - {compiler: clang++-21, standard: 23, mode: debug, arch: x86}
+          - {compiler: clang++-22, standard: 26, mode: release, arch: x86}
+          - {compiler: g++-15, standard: 26, mode: release, arch: arm}
+          - {compiler: g++-16, standard: 23, mode: release, arch: arm}
+          - {compiler: g++-16, standard: 26, mode: release, arch: x86}
+          - {compiler: g++-15, standard: 23, mode: release, arch: x86}
+          - {compiler: clang++-22, standard: 23, mode: sanitize, arch: arm}
+          - {compiler: clang++-21, standard: 26, mode: sanitize, arch: x86}
+          - {compiler: clang++-21, standard: 26, mode: debug, arch: arm}
           - {compiler: clang++-22, standard: 23, arch: x86, mode: dev}
           - {compiler: clang++-22, standard: 23, arch: x86, mode: release, enables: --enable-dpdk, options: --cook dpdk --dpdk-machine corei7-avx, info: 'dpdk, '}
           - {compiler: clang++-22, standard: 23, arch: x86, mode: debug, enables: --enable-cxx-modules, enable-ccache: false, info: 'modules, '}

CMakeLists.txt

@@ -145,7 +145,7 @@ set (CMAKE_CXX_FLAGS_SANITIZE
   FORCE)
 
 set (CMAKE_CXX_STANDARD
-  "23"
+  "26"
   CACHE
   STRING
   "C++ standard to build with.")

configure.py

@@ -244,7 +244,7 @@ def identify_best_standard(cpp_standards, compiler):
 
 
 if not args.cpp_standard:
-    cpp_standards = ['23', '20']
+    cpp_standards = ['26', '23']
     args.cpp_standard = identify_best_standard(cpp_standards, compiler=args.cxx)
 
 # Resolve compiler cache

cooking_recipe.cmake

@@ -309,8 +309,8 @@ cooking_ingredient (dpdk
 
 cooking_ingredient (fmt
   EXTERNAL_PROJECT_ARGS
-    URL https://github.com/fmtlib/fmt/archive/11.2.0.tar.gz
-    URL_MD5 2f3701cada629ca455c3388d1089f5bd
+    URL https://github.com/fmtlib/fmt/archive/12.2.0.tar.gz
+    URL_MD5 7be257d8bfad55bc9eabb1288ad895be
   CMAKE_ARGS
     -DFMT_DOC=OFF
     -DFMT_TEST=OFF)