diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index bfd4ca7287..7b80a91a16 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -199,7 +199,7 @@ jobs:
       run: |
         uv pip install --python=python --system setuptools
         pytest tests/extra_setuptools
-      if: "!(matrix.runs-on == 'windows-2022')"
+      if: matrix.runs-on != 'windows-2022'
 
   manylinux:
     name: Manylinux on 🐍 3.13t • GIL
@@ -330,9 +330,10 @@ jobs:
             container_suffix: "-bullseye"
           - 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:
@@ -348,6 +349,7 @@ 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
@@ -491,9 +493,9 @@ jobs:
           - { gcc: 9, std: 20 }
           - { gcc: 10, std: 17 }
           - { gcc: 10, 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:
@@ -515,6 +517,7 @@ 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 98ff61f5d7..08e26f71d0 100644
--- a/include/pybind11/detail/class.h
+++ b/include/pybind11/detail/class.h
@@ -499,8 +499,13 @@ 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/detail/function_record_pyobject.h b/include/pybind11/detail/function_record_pyobject.h
index 4cc8b242e8..13895b96f2 100644
--- a/include/pybind11/detail/function_record_pyobject.h
+++ b/include/pybind11/detail/function_record_pyobject.h
@@ -173,9 +173,6 @@ inline int tp_init_impl(PyObject *, PyObject *, PyObject *) {
     // return -1; // Unreachable.
 }
 
-// The implementation needs the definition of `class cpp_function`.
-void tp_dealloc_impl(PyObject *self);
-
 inline void tp_free_impl(void *) {
     pybind11_fail("UNEXPECTED CALL OF function_record_PyTypeObject_methods::tp_free_impl");
 }
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index d4f9a41e0c..d4ce1a492a 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -511,9 +511,14 @@ inline PyThreadState *get_thread_state_unchecked() {
 void keep_alive_impl(handle nurse, handle patient);
 inline PyObject *make_new_instance(PyTypeObject *type);
 
+PYBIND11_WARNING_PUSH
+PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
+
 // PYBIND11:REMINDER: Needs refactoring of existing pybind11 code.
 inline bool deregister_instance(instance *self, void *valptr, const type_info *tinfo);
 
+PYBIND11_WARNING_POP
+
 PYBIND11_NAMESPACE_BEGIN(smart_holder_type_caster_support)
 
 struct value_and_holder_helper {
diff --git a/include/pybind11/gil.h b/include/pybind11/gil.h
index 3f2c28f452..1a9bfeaddf 100644
--- a/include/pybind11/gil.h
+++ b/include/pybind11/gil.h
@@ -32,9 +32,14 @@ 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)
 
 /* The functions below essentially reproduce the PyGILState_* API using a RAII
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index e5e613df48..69976f745a 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -363,7 +363,7 @@ 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}