ci: verify both standard libraries as required gates

.github/workflows/linux.yml

@@ -81,6 +81,52 @@ jobs:
     - name: Test
       run: ./build/test/dross_test --gtest_color=yes
 
+  # The requirements say either standard library works across the whole Clang
+  # range, so libc++ is verified at both ends rather than left to best effort.
+  # Covering only the lower bound would leave the upper end of the declared
+  # range unverified against libc++ header changes and newly added warnings —
+  # and -Werror is public, so those reach consumers.
+  #
+  # libc++ is not part of the runner image, but it comes from the
+  # distribution's own repository, so no external source enters a required gate.
+  build-ubuntu-clang-libcxx:
+    name: Ubuntu Clang ${{ matrix.clang }} libc++ (${{ matrix.build_type }})
+    runs-on: ubuntu-26.04
+    strategy:
+      fail-fast: false
+      matrix:
+        build_type: [Debug, Release]
+        clang: [20, 22]
+
+    steps:
+    - uses: actions/checkout@v7
+
+    - name: Install libc++
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y \
+          libc++-${{ matrix.clang }}-dev libc++abi-${{ matrix.clang }}-dev
+
+    - name: Report toolchain versions
+      run: |
+        clang-${{ matrix.clang }} --version
+        cmake --version
+
+    - name: Configure CMake
+      run: |
+        cmake -S . -B build \
+          -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} \
+          -DCMAKE_C_COMPILER=clang-${{ matrix.clang }} \
+          -DCMAKE_CXX_COMPILER=clang++-${{ matrix.clang }} \
+          -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
+          -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"
+
+    - name: Build
+      run: cmake --build build --config ${{ matrix.build_type }} -j"$(nproc)"
+
+    - name: Test
+      run: ./build/test/dross_test --gtest_color=yes
+
   build-static-library:
     name: Static Library Build
     runs-on: ubuntu-24.04

.github/workflows/nightly.yml

@@ -98,41 +98,6 @@ jobs:
     - name: Test
       run: ./build/test/dross_test --gtest_color=yes
 
-  # The required gates build Clang against the default standard library.
-  # This covers the other choice, so neither ends up being the only one that
-  # works.
-  libcxx:
-    name: Clang with libc++
-    runs-on: ubuntu-26.04
-
-    steps:
-    - uses: actions/checkout@v7
-
-    - name: Install libc++
-      run: |
-        sudo apt-get update
-        sudo apt-get install -y libc++-20-dev libc++abi-20-dev
-
-    - name: Report toolchain versions
-      run: |
-        clang-20 --version
-        cmake --version
-
-    - name: Configure CMake
-      run: |
-        cmake -S . -B build \
-          -DCMAKE_BUILD_TYPE=Release \
-          -DCMAKE_C_COMPILER=clang-20 \
-          -DCMAKE_CXX_COMPILER=clang++-20 \
-          -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
-          -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"
-
-    - name: Build
-      run: cmake --build build -j"$(nproc)"
-
-    - name: Test
-      run: ./build/test/dross_test --gtest_color=yes
-
   floating-runners:
     name: Floating Runner (${{ matrix.os }})
     runs-on: ${{ matrix.os }}

README.md

@@ -24,7 +24,7 @@
 ### Requirements
 
 - **C++23** compatible compiler, verified in CI as:
-  - Linux: GCC 13–15 or Clang 20–22
+  - Linux: GCC 13–15, or Clang 20–22 with either libstdc++ or libc++
   - macOS: the Apple Clang shipped with macOS 15 or 26
   - Newer versions are best effort, exercised by the nightly toolchain watch
 - **CMake 3.20+**

docs/sphinx/source/contributing.rst

@@ -33,8 +33,9 @@ Development Setup
 Build Requirements
 ~~~~~~~~~~~~~~~~~~
 
-- C++23 compatible compiler: on Linux, GCC 13-15 or Clang 20-22; on macOS,
-  the Apple Clang shipped with macOS 15 or 26. Newer versions are best effort.
+- C++23 compatible compiler: on Linux, GCC 13-15, or Clang 20-22 with either
+  libstdc++ or libc++; on macOS, the Apple Clang shipped with macOS 15 or 26.
+  Newer versions are best effort.
 - CMake 3.20 or later
 - Git
 

docs/sphinx/source/getting-started.rst

@@ -10,17 +10,19 @@ To build and use dross, you need:
 
 - **C++ Compiler**: Supporting C++23 standard
 
-  - On Linux: GCC 13 through 15, or Clang 20 through 22
+  - On Linux: GCC 13 through 15, or Clang 20 through 22 with either libstdc++
+    or libc++
   - On macOS: the Apple Clang shipped with macOS 15 or 26
 
   Newer versions are best effort: they are exercised by the nightly toolchain
   watch rather than by the required build matrix.
 
   The Clang lower bound is higher than the GCC one because older Clang
   releases cannot compile this library's C++23 ``std::expected`` usage against
-  the libstdc++ they are paired with on Ubuntu 24.04, whether or not newer
-  libstdc++ headers are installed alongside. Either standard library works
-  from Clang 20 onwards, so neither is imposed.
+  the libstdc++ they are paired with on Ubuntu 24.04 — installing the
+  libstdc++ 14 headers alongside them does not change that either. From Clang
+  20 onwards both standard libraries work, and the build matrix verifies each
+  of them, so neither is imposed on you.
 
 - **Build System**: CMake 3.20 or later
 - **Operating System**: Linux or macOS