ci: add github actions jobs to test gcc and libc++

Author: ryanofsky

.github/workflows/ci.yml

@@ -8,16 +8,22 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    strategy:
+      fail-fast: false
+      matrix:
+        config: [gcc-libstdc++, clang-libc++]
+
+    name: build • ${{ matrix.config }}
+
     steps:
-    - uses: actions/checkout@v4
+      - uses: actions/checkout@v4
 
-    - name: Install Nix
-      uses: cachix/install-nix-action@v31       # 2025-05-27, from https://github.com/cachix/install-nix-action/tags
-      with:
-        nix_path: nixpkgs=channel:nixos-25.05   # latest release
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31       # 2025-05-27, from https://github.com/cachix/install-nix-action/tags
+        with:
+          nix_path: nixpkgs=channel:nixos-25.05   # latest release
 
-    - name: Configure, build, test
-      run: |
-        nix-shell --pure --run "ci/scripts/configure.sh"
-        nix-shell --pure --run "ci/scripts/build.sh"
-        nix-shell --pure --run "ci/scripts/test.sh"
+      - name: Run CI script
+        env:
+          CI_CONFIG: ci/configs/${{ matrix.config }}.sh
+        run: ci/run.sh

ci/configs/clang-libc++.sh

@@ -0,0 +1,12 @@
+NIX_ARGS=(
+  --arg libcxx true
+)
+CMAKE_ARGS=(
+  -DCMAKE_BUILD_TYPE=Debug
+  -DCMAKE_CXX_COMPILER=clang++
+  -DCMAKE_CXX_STANDARD_LIBRARY=libc++
+  -DCMAKE_CXX_FLAGS="-Werror -Wall -Wextra -Wpedantic -Wthread-safety-analysis -Wno-unused-parameter"
+  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
+  -DMP_ENABLE_CLANG_TIDY=ON
+  -DMP_ENABLE_IWYU=ON
+)

ci/configs/gcc-libstdc++.sh

@@ -0,0 +1,4 @@
+CMAKE_ARGS=(
+  -DCMAKE_BUILD_TYPE=Debug
+  -DCMAKE_CXX_FLAGS="-Werror -Wall -Wextra -Wpedantic -Wno-unused-parameter"
+)

ci/run.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -euxo pipefail
+
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+
+for stage in configure build test; do
+  nix-shell --pure --keep CI_CONFIG "${NIX_ARGS[@]+"${NIX_ARGS[@]}"}" --run ci/scripts/${stage}.sh shell.nix
+done

ci/scripts/build.sh

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
-set -euo pipefail
+set -euxo pipefail
 
 cmake --build build -t all tests mpexamples -- -k 0

ci/scripts/configure.sh

@@ -1,10 +1,6 @@
 #!/usr/bin/env bash
-set -euo pipefail
+set -euxo pipefail
 
-cmake -B build -G Ninja \
-  -DCMAKE_BUILD_TYPE=Debug \
-  -DCMAKE_CXX_COMPILER=clang++ \
-  -DCMAKE_CXX_FLAGS="-Wall -Wextra -Wpedantic -Wthread-safety-analysis -Wno-unused-parameter" \
-  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-  -DMP_ENABLE_CLANG_TIDY=ON \
-  -DMP_ENABLE_IWYU=ON
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+
+cmake -B build -G Ninja "${CMAKE_ARGS[@]+"${CMAKE_ARGS[@]}"}"

ci/scripts/test.sh

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
-set -euo pipefail
+set -euxo pipefail
 
 ctest --test-dir build --output-on-failure

shell.nix

@@ -1,14 +1,22 @@
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import <nixpkgs> {}
+, libcxx ? false # Whether to use libc++ toolchain and libraries instead of libstdc++
+}:
 
-pkgs.mkShell {
-  buildInputs = with pkgs; [
+let
+  lib  = pkgs.lib;
+  llvm = pkgs.llvmPackages_20;
+  mkShell = pkgs.mkShell.override (lib.optionalAttrs libcxx { stdenv = llvm.libcxxStdenv; });
+  capnproto = pkgs.capnproto.override (lib.optionalAttrs libcxx { clangStdenv = llvm.libcxxStdenv; });
+in mkShell {
+  buildInputs = [
     capnproto
+    llvm.libcxx
   ];
   nativeBuildInputs = with pkgs; [
     cmake
     include-what-you-use
-    llvmPackages_20.clang
-    llvmPackages_20.clang-tools
+    llvm.clang
+    llvm.clang-tools
     ninja
   ];
 }