diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e09540b3e8..75124412ec 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -217,7 +217,7 @@ jobs:
       run: |
         pip install setuptools
         pytest tests/extra_setuptools
-      if: "!(matrix.runs-on == 'windows-2022')"
+      if: matrix.runs-on != 'windows-2022'
 
   manylinux:
     name: Manylinux on 🐍 3.13t • GIL
@@ -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
@@ -521,11 +523,11 @@ jobs:
           - { gcc: 9, std: 20 }
           - { gcc: 10, std: 17 }
           - { gcc: 10, std: 20 }
-          - { gcc: 11, std: 20 }
+          - { gcc: 11, std: 20, cxx_flags: "-Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls" }
           - { gcc: 12, std: 20 }
-          - { gcc: 13, std: 20 }
+          - { gcc: 13, std: 20, cxx_flags: "-Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls" }
 
-    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }}• x64"
+    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }} • x64${{ matrix.cxx_flags && ' • cxx_flags' || '' }}"
     container: "gcc:${{ matrix.gcc }}"
 
     steps:
@@ -547,6 +549,7 @@ 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
diff --git a/include/pybind11/detail/class.h b/include/pybind11/detail/class.h
index 1ed2e90c2c..b26e76f6cf 100644
--- a/include/pybind11/detail/class.h
+++ b/include/pybind11/detail/class.h
@@ -497,8 +497,13 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
     Py_DECREF(type);
 }
 
+PYBIND11_WARNING_PUSH
+PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
+
 std::string error_string();
 
+PYBIND11_WARNING_POP
+
 /** Create the type which can be used as a common base for all classes.  This is
     needed in order to satisfy Python's requirements for multiple inheritance.
     Return value: New reference. */
diff --git a/include/pybind11/gil.h b/include/pybind11/gil.h
index 888810493c..255eaa9438 100644
--- a/include/pybind11/gil.h
+++ b/include/pybind11/gil.h
@@ -21,9 +21,14 @@ PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
+PYBIND11_WARNING_PUSH
+PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
+
 // forward declarations
 PyThreadState *get_thread_state_unchecked();
 
+PYBIND11_WARNING_POP
+
 PYBIND11_NAMESPACE_END(detail)
 
 #if !defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 315a5bad04..e5e333a846 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -344,7 +344,7 @@ elseif(MSVC)
 else()
   file(
     WRITE ${CMAKE_CURRENT_BINARY_DIR}/main.cpp
-    "#include <filesystem>\nint main(int argc, char ** argv) {\n  std::filesystem::path p(argv[0]);\n  return p.string().length();\n}"
+    "#include <filesystem>\nint main(int /*argc*/, char ** argv) {\n  std::filesystem::path p(argv[0]);\n  return p.string().length();\n}"
   )
   try_compile(
     STD_FS_NO_LIB_NEEDED ${CMAKE_CURRENT_BINARY_DIR}