CI: Fail on any warnings with clang 18

Author: rhaschke

.github/workflows/ci.yml

@@ -360,9 +360,10 @@ jobs:
             container_suffix: "-bookworm"
           - clang: 18
             std: 20
+            cxx_flags: "-Werror -Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls"
             container_suffix: "-bookworm"
 
-    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64"
+    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64${{ matrix.cxx_flags && ' • cxx_flags' || '' }}"
     container: "silkeh/clang:${{ matrix.clang }}${{ matrix.container_suffix }}"
 
     steps:
@@ -378,6 +379,7 @@ jobs:
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="${{ matrix.cxx_flags }}"
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
     - name: Build