diff --git a/.appveyor.yml b/.appveyor.yml
index 360760ac8d..934edfbd0b 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -2,6 +2,8 @@ version: 1.0.{build}
 image:
 - Visual Studio 2017
 test: off
+skip_commits:
+  message: /.*/  # Skip ALL commits.
 skip_branch_with_pr: true
 build:
   parallel: true
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
deleted file mode 100644
index 2c7d170839..0000000000
--- a/.github/dependabot.yml
+++ /dev/null
@@ -1,7 +0,0 @@
-version: 2
-updates:
-  # Maintain dependencies for GitHub Actions
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "daily"
diff --git a/.github/labeler.yml b/.github/labeler.yml
deleted file mode 100644
index abb0d05aaa..0000000000
--- a/.github/labeler.yml
+++ /dev/null
@@ -1,8 +0,0 @@
-docs:
-- any:
-  - 'docs/**/*.rst'
-  - '!docs/changelog.rst'
-  - '!docs/upgrade.rst'
-
-ci:
-- '.github/workflows/*.yml'
diff --git a/.github/labeler_merged.yml b/.github/labeler_merged.yml
deleted file mode 100644
index 2374ad42e4..0000000000
--- a/.github/labeler_merged.yml
+++ /dev/null
@@ -1,3 +0,0 @@
-needs changelog:
-- all:
-  - '!docs/changelog.rst'
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
deleted file mode 100644
index a11cae1ab0..0000000000
--- a/.github/workflows/ci.yml
+++ /dev/null
@@ -1,969 +0,0 @@
-name: CI
-
-on:
-  workflow_dispatch:
-  pull_request:
-  push:
-    branches:
-      - master
-      - stable
-      - v*
-
-concurrency:
-  group: test-${{ github.ref }}
-  cancel-in-progress: true
-
-env:
-  PIP_ONLY_BINARY: numpy
-  FORCE_COLOR: 3
-  PYTEST_TIMEOUT: 300
-
-jobs:
-  # This is the "main" test suite, which tests a large number of different
-  # versions of default compilers and Python versions in GitHub Actions.
-  standard:
-    strategy:
-      fail-fast: false
-      matrix:
-        runs-on: [ubuntu-latest, windows-2022, macos-latest]
-        python:
-        - '3.6'
-        - '3.9'
-        - '3.10'
-        - '3.11'
-        - 'pypy-3.7'
-        - 'pypy-3.8'
-        - 'pypy-3.9'
-
-        # Items in here will either be added to the build matrix (if not
-        # present), or add new keys to an existing matrix element if all the
-        # existing keys match.
-        #
-        # We support an optional key: args, for cmake args
-        include:
-          # Just add a key
-          - runs-on: ubuntu-latest
-            python: '3.6'
-            args: >
-              -DPYBIND11_FINDPYTHON=ON
-              -DCMAKE_CXX_FLAGS="-D_=1"
-          - runs-on: ubuntu-latest
-            python: 'pypy-3.8'
-            args: >
-              -DPYBIND11_FINDPYTHON=ON
-          - runs-on: windows-2019
-            python: '3.6'
-            args: >
-              -DPYBIND11_FINDPYTHON=ON
-          # Inject a couple Windows 2019 runs
-          - runs-on: windows-2019
-            python: '3.9'
-
-    name: "🐍 ${{ matrix.python }} • ${{ matrix.runs-on }} • x64 ${{ matrix.args }}"
-    runs-on: ${{ matrix.runs-on }}
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python ${{ matrix.python }}
-      uses: actions/setup-python@v4
-      with:
-        python-version: ${{ matrix.python }}
-
-    - name: Setup Boost (Linux)
-      # Can't use boost + define _
-      if: runner.os == 'Linux' && matrix.python != '3.6'
-      run: sudo apt-get install libboost-dev
-
-    - name: Setup Boost (macOS)
-      if: runner.os == 'macOS'
-      run: brew install boost
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Cache wheels
-      if: runner.os == 'macOS'
-      uses: actions/cache@v3
-      with:
-        # This path is specific to macOS - we really only need it for PyPy NumPy wheels
-        # See https://github.com/actions/cache/blob/master/examples.md#python---pip
-        # for ways to do this more generally
-        path: ~/Library/Caches/pip
-        # Look to see if there is a cache hit for the corresponding requirements file
-        key: ${{ runner.os }}-pip-${{ matrix.python }}-x64-${{ hashFiles('tests/requirements.txt') }}
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    - name: Setup annotations on Linux
-      if: runner.os == 'Linux'
-      run: python -m pip install pytest-github-actions-annotate-failures
-
-    # First build - C++11 mode and inplace
-    # More-or-less randomly adding -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON here.
-    - name: Configure C++11 ${{ matrix.args }}
-      run: >
-        cmake -S . -B .
-        -DPYBIND11_WERROR=ON
-        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=11
-        ${{ matrix.args }}
-
-    - name: Build C++11
-      run: cmake --build . -j 2
-
-    - name: Python tests C++11
-      run: cmake --build . --target pytest -j 2
-
-    - name: C++11 tests
-      # TODO: Figure out how to load the DLL on Python 3.8+
-      if: "!(runner.os == 'Windows' && (matrix.python == 3.8 || matrix.python == 3.9 || matrix.python == '3.10' || matrix.python == '3.11' || matrix.python == 'pypy-3.8'))"
-      run: cmake --build .  --target cpptest -j 2
-
-    - name: Interface test C++11
-      run: cmake --build . --target test_cmake_build
-
-    - name: Clean directory
-      run: git clean -fdx
-
-    # Second build - C++17 mode and in a build directory
-    # More-or-less randomly adding -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF here.
-    - name: Configure C++17
-      run: >
-        cmake -S . -B build2
-        -DPYBIND11_WERROR=ON
-        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-        ${{ matrix.args }}
-
-    - name: Build
-      run: cmake --build build2 -j 2
-
-    - name: Python tests
-      run: cmake --build build2 --target pytest
-
-    - name: C++ tests
-      # TODO: Figure out how to load the DLL on Python 3.8+
-      if: "!(runner.os == 'Windows' && (matrix.python == 3.8 || matrix.python == 3.9 || matrix.python == '3.10' || matrix.python == '3.11' || matrix.python == 'pypy-3.8'))"
-      run: cmake --build build2 --target cpptest
-
-    # Third build - C++17 mode with unstable ABI
-    - name: Configure (unstable ABI)
-      run: >
-        cmake -S . -B build3
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-        -DPYBIND11_INTERNALS_VERSION=10000000
-        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
-        ${{ matrix.args }}
-
-    - name: Build (unstable ABI)
-      run: cmake --build build3 -j 2
-
-    - name: Python tests (unstable ABI)
-      run: cmake --build build3 --target pytest
-
-    - name: Interface test
-      run: cmake --build build2 --target test_cmake_build
-
-    # This makes sure the setup_helpers module can build packages using
-    # setuptools
-    - name: Setuptools helpers test
-      run: pytest tests/extra_setuptools
-      if: "!(matrix.runs-on == 'windows-2022')"
-
-
-  deadsnakes:
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-        # TODO: Fails on 3.10, investigate
-        - python-version: "3.9"
-          python-debug: true
-          valgrind: true
-        - python-version: "3.11"
-          python-debug: false
-
-    name: "🐍 ${{ matrix.python-version }}${{ matrix.python-debug && '-dbg' || '' }} (deadsnakes)${{ matrix.valgrind && ' • Valgrind' || '' }} • x64"
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python ${{ matrix.python-version }} (deadsnakes)
-      uses: deadsnakes/action@v2.1.1
-      with:
-        python-version: ${{ matrix.python-version }}
-        debug: ${{ matrix.python-debug }}
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Valgrind cache
-      if: matrix.valgrind
-      uses: actions/cache@v3
-      id: cache-valgrind
-      with:
-        path: valgrind
-        key: 3.16.1 # Valgrind version
-
-    - name: Compile Valgrind
-      if: matrix.valgrind && steps.cache-valgrind.outputs.cache-hit != 'true'
-      run: |
-        VALGRIND_VERSION=3.16.1
-        curl https://sourceware.org/pub/valgrind/valgrind-$VALGRIND_VERSION.tar.bz2 -o - | tar xj
-        mv valgrind-$VALGRIND_VERSION valgrind
-        cd valgrind
-        ./configure
-        make -j 2 > /dev/null
-
-    - name: Install Valgrind
-      if: matrix.valgrind
-      working-directory: valgrind
-      run: |
-        sudo make install
-        sudo apt-get update
-        sudo apt-get install libc6-dbg  # Needed by Valgrind
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    - name: Configure
-      env:
-        SETUPTOOLS_USE_DISTUTILS: stdlib
-      run: >
-        cmake -S . -B build
-        -DCMAKE_BUILD_TYPE=Debug
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-
-    - name: Build
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-    - name: C++ tests
-      run: cmake --build build --target cpptest
-
-    - name: Run Valgrind on Python tests
-      if: matrix.valgrind
-      run: cmake --build build --target memcheck
-
-
-  # Testing on clang using the excellent silkeh clang docker images
-  clang:
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        clang:
-          - 3.6
-          - 3.7
-          - 3.9
-          - 7
-          - 9
-          - dev
-        std:
-          - 11
-        include:
-          - clang: 5
-            std: 14
-          - clang: 10
-            std: 20
-          - clang: 10
-            std: 17
-          - clang: 11
-            std: 20
-          - clang: 12
-            std: 20
-          - clang: 13
-            std: 20
-          - clang: 14
-            std: 20
-
-    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64"
-    container: "silkeh/clang:${{ matrix.clang }}"
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Add wget and python3
-      run: apt-get update && apt-get install -y python3-dev python3-numpy python3-pytest libeigen3-dev
-
-    - name: Configure
-      shell: bash
-      run: >
-        cmake -S . -B build
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-    - name: C++ tests
-      run: cmake --build build --target cpptest
-
-    - name: Interface test
-      run: cmake --build build --target test_cmake_build
-
-
-  # Testing NVCC; forces sources to behave like .cu files
-  cuda:
-    runs-on: ubuntu-latest
-    name: "🐍 3.10 • CUDA 11.7 • Ubuntu 22.04"
-    container: nvidia/cuda:11.7.0-devel-ubuntu22.04
-
-    steps:
-    - uses: actions/checkout@v3
-
-    # tzdata will try to ask for the timezone, so set the DEBIAN_FRONTEND
-    - name: Install 🐍 3
-      run: apt-get update && DEBIAN_FRONTEND="noninteractive" apt-get install -y cmake git python3-dev python3-pytest python3-numpy
-
-    - name: Configure
-      run: cmake -S . -B build -DPYBIND11_CUDA_TESTS=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
-
-    - name: Build
-      run: cmake --build build -j2 --verbose
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-
-# TODO: Internal compiler error - report to NVidia
-#  # Testing CentOS 8 + PGI compilers
-#  centos-nvhpc8:
-#    runs-on: ubuntu-latest
-#    name: "🐍 3 • CentOS8 / PGI 20.11 • x64"
-#    container: centos:8
-#
-#    steps:
-#    - uses: actions/checkout@v3
-#
-#    - name: Add Python 3 and a few requirements
-#      run: yum update -y && yum install -y git python3-devel python3-numpy python3-pytest make environment-modules
-#
-#    - name: Install CMake with pip
-#      run: |
-#        python3 -m pip install --upgrade pip
-#        python3 -m pip install cmake --prefer-binary
-#
-#    - name: Install NVidia HPC SDK
-#      run: >
-#        yum -y install
-#        https://developer.download.nvidia.com/hpc-sdk/20.11/nvhpc-20-11-20.11-1.x86_64.rpm
-#        https://developer.download.nvidia.com/hpc-sdk/20.11/nvhpc-2020-20.11-1.x86_64.rpm
-#
-#    - name: Configure
-#      shell: bash
-#      run: |
-#        source /etc/profile.d/modules.sh
-#        module load /opt/nvidia/hpc_sdk/modulefiles/nvhpc/20.11
-#        cmake -S . -B build -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_STANDARD=14 -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-#
-#    - name: Build
-#      run: cmake --build build -j 2 --verbose
-#
-#    - name: Python tests
-#      run: cmake --build build --target pytest
-#
-#    - name: C++ tests
-#      run: cmake --build build --target cpptest
-#
-#    - name: Interface test
-#      run: cmake --build build --target test_cmake_build
-
-
-  # Testing on CentOS 7 + PGI compilers, which seems to require more workarounds
-  centos-nvhpc7:
-    runs-on: ubuntu-latest
-    name: "🐍 3 • CentOS7 / PGI 22.9 • x64"
-    container: centos:7
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Add Python 3 and a few requirements
-      run: yum update -y && yum install -y epel-release && yum install -y git python3-devel make environment-modules cmake3 yum-utils
-
-    - name: Install NVidia HPC SDK
-      run: yum-config-manager --add-repo https://developer.download.nvidia.com/hpc-sdk/rhel/nvhpc.repo && yum -y install nvhpc-22.9
-
-    # On CentOS 7, we have to filter a few tests (compiler internal error)
-    # and allow deeper template recursion (not needed on CentOS 8 with a newer
-    # standard library). On some systems, you many need further workarounds:
-    # https://github.com/pybind/pybind11/pull/2475
-    - name: Configure
-      shell: bash
-      run: |
-        source /etc/profile.d/modules.sh
-        module load /opt/nvidia/hpc_sdk/modulefiles/nvhpc/22.9
-        cmake3 -S . -B build -DDOWNLOAD_CATCH=ON \
-                            -DCMAKE_CXX_STANDARD=11 \
-                            -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)") \
-                            -DCMAKE_CXX_FLAGS="-Wc,--pending_instantiations=0" \
-                            -DPYBIND11_TEST_FILTER="test_smart_ptr.cpp"
-
-    # Building before installing Pip should produce a warning but not an error
-    - name: Build
-      run: cmake3 --build build -j 2 --verbose
-
-    - name: Install CMake with pip
-      run: |
-        python3 -m pip install --upgrade pip
-        python3 -m pip install pytest
-
-    - name: Python tests
-      run: cmake3 --build build --target pytest
-
-    - name: C++ tests
-      run: cmake3 --build build --target cpptest
-
-    - name: Interface test
-      run: cmake3 --build build --target test_cmake_build
-
-
-  # Testing on GCC using the GCC docker images (only recent images supported)
-  gcc:
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - { gcc: 7, std: 11 }
-          - { gcc: 7, std: 17 }
-          - { gcc: 8, std: 14 }
-          - { gcc: 8, std: 17 }
-          - { gcc: 10, std: 17 }
-          - { gcc: 11, std: 20 }
-          - { gcc: 12, std: 20 }
-
-    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }}• x64"
-    container: "gcc:${{ matrix.gcc }}"
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Add Python 3
-      run: apt-get update; apt-get install -y python3-dev python3-numpy python3-pytest python3-pip libeigen3-dev
-
-    - name: Update pip
-      run: python3 -m pip install --upgrade pip
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Configure
-      shell: bash
-      run: >
-        cmake -S . -B build
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-    - name: C++ tests
-      run: cmake --build build --target cpptest
-
-    - name: Interface test
-      run: cmake --build build --target test_cmake_build
-
-
-  # Testing on ICC using the oneAPI apt repo
-  icc:
-    runs-on: ubuntu-20.04
-    strategy:
-      fail-fast: false
-
-    name: "🐍 3 • ICC latest • x64"
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Add apt repo
-      run: |
-        sudo apt-get update
-        sudo apt-get install -y wget build-essential pkg-config cmake ca-certificates gnupg
-        wget https://apt.repos.intel.com/intel-gpg-keys/GPG-PUB-KEY-INTEL-SW-PRODUCTS-2023.PUB
-        sudo apt-key add GPG-PUB-KEY-INTEL-SW-PRODUCTS-2023.PUB
-        echo "deb https://apt.repos.intel.com/oneapi all main" | sudo tee /etc/apt/sources.list.d/oneAPI.list
-
-    - name: Add ICC & Python 3
-      run: sudo apt-get update; sudo apt-get install -y intel-oneapi-compiler-dpcpp-cpp-and-cpp-classic cmake python3-dev python3-numpy python3-pytest python3-pip
-
-    - name: Update pip
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        python3 -m pip install --upgrade pip
-
-    - name: Install dependencies
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        python3 -m pip install -r tests/requirements.txt
-
-    - name: Configure C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake -S . -B build-11     \
-        -DPYBIND11_WERROR=ON    \
-        -DDOWNLOAD_CATCH=ON     \
-        -DDOWNLOAD_EIGEN=OFF    \
-        -DCMAKE_CXX_STANDARD=11             \
-        -DCMAKE_CXX_COMPILER=$(which icpc)  \
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 -j 2 -v
-
-    - name: Python tests C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        sudo service apport stop
-        cmake --build build-11 --target check
-
-    - name: C++ tests C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 --target cpptest
-
-    - name: Interface test C++11
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-11 --target test_cmake_build
-
-    - name: Configure C++17
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake -S . -B build-17     \
-        -DPYBIND11_WERROR=ON    \
-        -DDOWNLOAD_CATCH=ON     \
-        -DDOWNLOAD_EIGEN=OFF    \
-        -DCMAKE_CXX_STANDARD=17             \
-        -DCMAKE_CXX_COMPILER=$(which icpc)  \
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build C++17
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-17 -j 2 -v
-
-    - name: Python tests C++17
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        sudo service apport stop
-        cmake --build build-17 --target check
-
-    - name: C++ tests C++17
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-17 --target cpptest
-
-    - name: Interface test C++17
-      run: |
-        set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-17 --target test_cmake_build
-
-
-  # Testing on CentOS (manylinux uses a centos base, and this is an easy way
-  # to get GCC 4.8, which is the manylinux1 compiler).
-  centos:
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        container:
-          - "centos:7"  # GCC 4.8
-          - "almalinux:8"
-          - "almalinux:9"
-
-    name: "🐍 3 • ${{ matrix.container }} • x64"
-    container: "${{ matrix.container }}"
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Add Python 3 (RHEL 7)
-      if: matrix.container == 'centos:7'
-      run: yum update -y && yum install -y python3-devel gcc-c++ make git
-
-    - name: Add Python 3 (RHEL 8+)
-      if: matrix.container != 'centos:7'
-      run: dnf update -y && dnf install -y python3-devel gcc-c++ make git
-
-    - name: Update pip
-      run: python3 -m pip install --upgrade pip
-
-    - name: Install dependencies
-      run: |
-        python3 -m pip install cmake -r tests/requirements.txt
-
-    - name: Configure
-      shell: bash
-      run: >
-        cmake -S . -B build
-        -DCMAKE_BUILD_TYPE=MinSizeRel
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=11
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Build
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-    - name: C++ tests
-      run: cmake --build build --target cpptest
-
-    - name: Interface test
-      run: cmake --build build --target test_cmake_build
-
-
-  # This tests an "install" with the CMake tools
-  install-classic:
-    name: "🐍 3.7 • Debian • x86 •  Install"
-    runs-on: ubuntu-latest
-    container: i386/debian:buster
-
-    steps:
-    - uses: actions/checkout@v1  # Required to run inside docker
-
-    - name: Install requirements
-      run: |
-        apt-get update
-        apt-get install -y git make cmake g++ libeigen3-dev python3-dev python3-pip
-        pip3 install "pytest==6.*"
-
-    - name: Configure for install
-      run: >
-        cmake .
-        -DPYBIND11_INSTALL=1 -DPYBIND11_TEST=0
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-
-    - name: Make and install
-      run: make install
-
-    - name: Copy tests to new directory
-      run: cp -a tests /pybind11-tests
-
-    - name: Make a new test directory
-      run: mkdir /build-tests
-
-    - name: Configure tests
-      run: >
-        cmake ../pybind11-tests
-        -DDOWNLOAD_CATCH=ON
-        -DPYBIND11_WERROR=ON
-        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
-      working-directory: /build-tests
-
-    - name: Python tests
-      run: make pytest -j 2
-      working-directory: /build-tests
-
-
-  # This verifies that the documentation is not horribly broken, and does a
-  # basic validation check on the SDist.
-  doxygen:
-    name: "Documentation build test"
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - uses: actions/setup-python@v4
-      with:
-        python-version: "3.x"
-
-    - name: Install Doxygen
-      run: sudo apt-get install -y doxygen librsvg2-bin # Changed to rsvg-convert in 20.04
-
-    - name: Build docs
-      run: pipx run nox -s docs
-
-    - name: Make SDist
-      run: pipx run nox -s build -- --sdist
-
-    - run: git status --ignored
-
-    - name: Check local include dir
-      run: >
-        ls pybind11;
-        python3 -c "import pybind11, pathlib; assert (a := pybind11.get_include()) == (b := str(pathlib.Path('include').resolve())), f'{a} != {b}'"
-
-    - name: Compare Dists (headers only)
-      working-directory: include
-      run: |
-        python3 -m pip install --user -U ../dist/*.tar.gz
-        installed=$(python3 -c "import pybind11; print(pybind11.get_include() + '/pybind11')")
-        diff -rq $installed ./pybind11
-
-  win32:
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-        - 3.6
-        - 3.7
-        - 3.8
-        - 3.9
-
-        include:
-          - python: 3.9
-            args: -DCMAKE_CXX_STANDARD=20
-          - python: 3.8
-            args: -DCMAKE_CXX_STANDARD=17
-          - python: 3.7
-            args: -DCMAKE_CXX_STANDARD=14
-
-
-    name: "🐍 ${{ matrix.python }} • MSVC 2019 • x86 ${{ matrix.args }}"
-    runs-on: windows-2019
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python ${{ matrix.python }}
-      uses: actions/setup-python@v4
-      with:
-        python-version: ${{ matrix.python }}
-        architecture: x86
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Prepare MSVC
-      uses: ilammy/msvc-dev-cmd@v1.12.0
-      with:
-        arch: x86
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    # First build - C++11 mode and inplace
-    - name: Configure ${{ matrix.args }}
-      run: >
-        cmake -S . -B build
-        -G "Visual Studio 16 2019" -A Win32
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        ${{ matrix.args }}
-    - name: Build C++11
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build -t pytest
-
-  win32-debug:
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-        - 3.8
-        - 3.9
-
-        include:
-          - python: 3.9
-            args: -DCMAKE_CXX_STANDARD=20
-          - python: 3.8
-            args: -DCMAKE_CXX_STANDARD=17
-
-    name: "🐍 ${{ matrix.python }} • MSVC 2019 (Debug) • x86 ${{ matrix.args }}"
-    runs-on: windows-2019
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python ${{ matrix.python }}
-      uses: actions/setup-python@v4
-      with:
-        python-version: ${{ matrix.python }}
-        architecture: x86
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Prepare MSVC
-      uses: ilammy/msvc-dev-cmd@v1.12.0
-      with:
-        arch: x86
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    # First build - C++11 mode and inplace
-    - name: Configure ${{ matrix.args }}
-      run: >
-        cmake -S . -B build
-        -G "Visual Studio 16 2019" -A Win32
-        -DCMAKE_BUILD_TYPE=Debug
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        ${{ matrix.args }}
-    - name: Build C++11
-      run: cmake --build build --config Debug -j 2
-
-    - name: Python tests
-      run: cmake --build build --config Debug -t pytest
-
-
-  windows-2022:
-    strategy:
-      fail-fast: false
-      matrix:
-        python:
-        - 3.9
-
-    name: "🐍 ${{ matrix.python }} • MSVC 2022 C++20 • x64"
-    runs-on: windows-2022
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python ${{ matrix.python }}
-      uses: actions/setup-python@v4
-      with:
-        python-version: ${{ matrix.python }}
-
-    - name: Prepare env
-      run: |
-        python3 -m pip install -r tests/requirements.txt
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Configure C++20
-      run: >
-        cmake -S . -B build
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=20
-
-    - name: Build C++20
-      run: cmake --build build -j 2
-
-    - name: Python tests
-      run: cmake --build build --target pytest
-
-    - name: C++20 tests
-      run: cmake --build build --target cpptest -j 2
-
-    - name: Interface test C++20
-      run: cmake --build build --target test_cmake_build
-
-  mingw:
-    name: "🐍 3 • windows-latest • ${{ matrix.sys }}"
-    runs-on: windows-latest
-    defaults:
-      run:
-        shell: msys2 {0}
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - { sys: mingw64, env: x86_64 }
-          - { sys: mingw32, env: i686 }
-    steps:
-    - uses: msys2/setup-msys2@v2
-      with:
-        msystem: ${{matrix.sys}}
-        install: >-
-          git
-          mingw-w64-${{matrix.env}}-gcc
-          mingw-w64-${{matrix.env}}-python-pip
-          mingw-w64-${{matrix.env}}-python-numpy
-          mingw-w64-${{matrix.env}}-python-scipy
-          mingw-w64-${{matrix.env}}-cmake
-          mingw-w64-${{matrix.env}}-make
-          mingw-w64-${{matrix.env}}-python-pytest
-          mingw-w64-${{matrix.env}}-eigen3
-          mingw-w64-${{matrix.env}}-boost
-          mingw-w64-${{matrix.env}}-catch
-
-    - uses: actions/checkout@v3
-
-    - name: Configure C++11
-      # LTO leads to many undefined reference like
-      # `pybind11::detail::function_call::function_call(pybind11::detail::function_call&&)
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build
-
-    - name: Build C++11
-      run: cmake --build build -j 2
-
-    - name: Python tests C++11
-      run: cmake --build build --target pytest -j 2
-
-    - name: C++11 tests
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build --target cpptest -j 2
-
-    - name: Interface test C++11
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build --target test_cmake_build
-
-    - name: Clean directory
-      run: git clean -fdx
-
-    - name: Configure C++14
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build2
-
-    - name: Build C++14
-      run: cmake --build build2 -j 2
-
-    - name: Python tests C++14
-      run: cmake --build build2 --target pytest -j 2
-
-    - name: C++14 tests
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target cpptest -j 2
-
-    - name: Interface test C++14
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target test_cmake_build
-
-    - name: Clean directory
-      run: git clean -fdx
-
-    - name: Configure C++17
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build3
-
-    - name: Build C++17
-      run: cmake --build build3 -j 2
-
-    - name: Python tests C++17
-      run: cmake --build build3 --target pytest -j 2
-
-    - name: C++17 tests
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target cpptest -j 2
-
-    - name: Interface test C++17
-      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build3 --target test_cmake_build
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
deleted file mode 100644
index 5ec0dd462f..0000000000
--- a/.github/workflows/configure.yml
+++ /dev/null
@@ -1,80 +0,0 @@
-name: Config
-
-on:
-  workflow_dispatch:
-  pull_request:
-  push:
-    branches:
-      - master
-      - stable
-      - v*
-
-jobs:
-  # This tests various versions of CMake in various combinations, to make sure
-  # the configure step passes.
-  cmake:
-    strategy:
-      fail-fast: false
-      matrix:
-        runs-on: [ubuntu-latest, macos-latest, windows-latest]
-        arch: [x64]
-        cmake: ["3.23"]
-
-        include:
-        - runs-on: ubuntu-latest
-          arch: x64
-          cmake: 3.4
-
-        - runs-on: macos-latest
-          arch: x64
-          cmake: 3.7
-
-        - runs-on: windows-2019
-          arch: x64 # x86 compilers seem to be missing on 2019 image
-          cmake: 3.18
-
-    name: 🐍 3.7 • CMake ${{ matrix.cmake }} • ${{ matrix.runs-on }}
-    runs-on: ${{ matrix.runs-on }}
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python 3.7
-      uses: actions/setup-python@v4
-      with:
-        python-version: 3.7
-        architecture: ${{ matrix.arch }}
-
-    - name: Prepare env
-      run: python -m pip install -r tests/requirements.txt
-
-    # An action for adding a specific version of CMake:
-    #   https://github.com/jwlawson/actions-setup-cmake
-    - name: Setup CMake ${{ matrix.cmake }}
-      uses: jwlawson/actions-setup-cmake@v1.13
-      with:
-        cmake-version: ${{ matrix.cmake }}
-
-    # These steps use a directory with a space in it intentionally
-    - name: Make build directories
-      run: mkdir "build dir"
-
-    - name: Configure
-      working-directory: build dir
-      shell: bash
-      run: >
-        cmake ..
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
-
-    # Only build and test if this was manually triggered in the GitHub UI
-    - name: Build
-      working-directory: build dir
-      if: github.event_name == 'workflow_dispatch'
-      run: cmake --build . --config Release
-
-    - name: Test
-      working-directory: build dir
-      if: github.event_name == 'workflow_dispatch'
-      run: cmake --build . --config Release --target check
diff --git a/.github/workflows/format.yml b/.github/workflows/format.yml
deleted file mode 100644
index 31d893c479..0000000000
--- a/.github/workflows/format.yml
+++ /dev/null
@@ -1,55 +0,0 @@
-# This is a format job. Pre-commit has a first-party GitHub action, so we use
-# that: https://github.com/pre-commit/action
-
-name: Format
-
-on:
-  workflow_dispatch:
-  pull_request:
-  push:
-    branches:
-    - master
-    - stable
-    - "v*"
-
-env:
-  FORCE_COLOR: 3
-
-jobs:
-  pre-commit:
-    name: Format
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v3
-    - uses: actions/setup-python@v4
-      with:
-        python-version: "3.x"
-    - name: Add matchers
-      run: echo "::add-matcher::$GITHUB_WORKSPACE/.github/matchers/pylint.json"
-    - uses: pre-commit/action@v3.0.0
-      with:
-        # Slow hooks are marked with manual - slow is okay here, run them too
-        extra_args: --hook-stage manual --all-files
-
-  clang-tidy:
-    # When making changes here, please also review the "Clang-Tidy" section
-    # in .github/CONTRIBUTING.md and update as needed.
-    name: Clang-Tidy
-    runs-on: ubuntu-latest
-    container: silkeh/clang:13
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Install requirements
-      run: apt-get update && apt-get install -y python3-dev python3-pytest
-
-    - name: Configure
-      run: >
-        cmake -S . -B build
-        -DCMAKE_CXX_CLANG_TIDY="$(which clang-tidy);--use-color;--warnings-as-errors=*"
-        -DDOWNLOAD_EIGEN=ON
-        -DDOWNLOAD_CATCH=ON
-        -DCMAKE_CXX_STANDARD=17
-
-    - name: Build
-      run: cmake --build build -j 2 -- --keep-going
diff --git a/.github/workflows/labeler.yml b/.github/workflows/labeler.yml
deleted file mode 100644
index 165a2fd87b..0000000000
--- a/.github/workflows/labeler.yml
+++ /dev/null
@@ -1,20 +0,0 @@
-name: Labeler
-on:
-  pull_request_target:
-    types: [closed]
-
-jobs:
-  label:
-    name: Labeler
-    runs-on: ubuntu-latest
-    steps:
-
-    - uses: actions/labeler@main
-      if: >
-        github.event.pull_request.merged == true &&
-        !startsWith(github.event.pull_request.title, 'chore(deps):') &&
-        !startsWith(github.event.pull_request.title, 'ci(fix):') &&
-        !startsWith(github.event.pull_request.title, 'docs(changelog):')
-      with:
-        repo-token: ${{ secrets.GITHUB_TOKEN }}
-        configuration-path: .github/labeler_merged.yml
diff --git a/.github/workflows/pip.yml b/.github/workflows/pip.yml
deleted file mode 100644
index f03a397019..0000000000
--- a/.github/workflows/pip.yml
+++ /dev/null
@@ -1,110 +0,0 @@
-name: Pip
-
-on:
-  workflow_dispatch:
-  pull_request:
-  push:
-    branches:
-    - master
-    - stable
-    - v*
-  release:
-    types:
-    - published
-
-env:
-  PIP_ONLY_BINARY: numpy
-
-jobs:
-  # This builds the sdists and wheels and makes sure the files are exactly as
-  # expected. Using Windows and Python 3.6, since that is often the most
-  # challenging matrix element.
-  test-packaging:
-    name: 🐍 3.6 • 📦 tests • windows-latest
-    runs-on: windows-latest
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup 🐍 3.6
-      uses: actions/setup-python@v4
-      with:
-        python-version: 3.6
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    - name: Python Packaging tests
-      run: pytest tests/extra_python_package/
-
-
-  # This runs the packaging tests and also builds and saves the packages as
-  # artifacts.
-  packaging:
-    name: 🐍 3.8 • 📦 & 📦 tests • ubuntu-latest
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup 🐍 3.8
-      uses: actions/setup-python@v4
-      with:
-        python-version: 3.8
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt build twine
-
-    - name: Python Packaging tests
-      run: pytest tests/extra_python_package/
-
-    - name: Build SDist and wheels
-      run: |
-        python -m build
-        PYBIND11_GLOBAL_SDIST=1 python -m build
-
-    - name: Check metadata
-      run: twine check dist/*
-
-    - name: Save standard package
-      uses: actions/upload-artifact@v3
-      with:
-        name: standard
-        path: dist/pybind11-*
-
-    - name: Save global package
-      uses: actions/upload-artifact@v3
-      with:
-        name: global
-        path: dist/pybind11_global-*
-
-
-
-  # When a GitHub release is made, upload the artifacts to PyPI
-  upload:
-    name: Upload to PyPI
-    runs-on: ubuntu-latest
-    if: github.event_name == 'release' && github.event.action == 'published'
-    needs: [packaging]
-
-    steps:
-    - uses: actions/setup-python@v4
-      with:
-        python-version: "3.x"
-
-    # Downloads all to directories matching the artifact names
-    - uses: actions/download-artifact@v3
-
-    - name: Publish standard package
-      uses: pypa/gh-action-pypi-publish@v1.5.1
-      with:
-        password: ${{ secrets.pypi_password }}
-        packages_dir: standard/
-
-    - name: Publish global package
-      uses: pypa/gh-action-pypi-publish@v1.5.1
-      with:
-        password: ${{ secrets.pypi_password_global }}
-        packages_dir: global/
diff --git a/.github/workflows/upstream.yml b/.github/workflows/upstream.yml
deleted file mode 100644
index 366284acf4..0000000000
--- a/.github/workflows/upstream.yml
+++ /dev/null
@@ -1,112 +0,0 @@
-
-name: Upstream
-
-on:
-  workflow_dispatch:
-  pull_request:
-
-concurrency:
-  group: upstream-${{ github.ref }}
-  cancel-in-progress: true
-
-env:
-  PIP_ONLY_BINARY: numpy
-
-jobs:
-  standard:
-    name: "🐍 3.11 latest internals • ubuntu-latest • x64"
-    runs-on: ubuntu-latest
-    if: "contains(github.event.pull_request.labels.*.name, 'python dev')"
-
-    steps:
-    - uses: actions/checkout@v3
-
-    - name: Setup Python 3.11
-      uses: actions/setup-python@v4
-      with:
-        python-version: "3.11-dev"
-
-    - name: Setup Boost (Linux)
-      if: runner.os == 'Linux'
-      run: sudo apt-get install libboost-dev
-
-    - name: Update CMake
-      uses: jwlawson/actions-setup-cmake@v1.13
-
-    - name: Prepare env
-      run: |
-        python -m pip install -r tests/requirements.txt
-
-    - name: Setup annotations on Linux
-      if: runner.os == 'Linux'
-      run: python -m pip install pytest-github-actions-annotate-failures
-
-    # First build - C++11 mode and inplace
-    - name: Configure C++11
-      run: >
-        cmake -S . -B .
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=11
-
-    - name: Build C++11
-      run: cmake --build . -j 2
-
-    - name: Python tests C++11
-      run: cmake --build . --target pytest -j 2
-
-    - name: C++11 tests
-      run: cmake --build .  --target cpptest -j 2
-
-    - name: Interface test C++11
-      run: cmake --build . --target test_cmake_build
-
-    - name: Clean directory
-      run: git clean -fdx
-
-    # Second build - C++17 mode and in a build directory
-    - name: Configure C++17
-      run: >
-        cmake -S . -B build2
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-        ${{ matrix.args }}
-        ${{ matrix.args2 }}
-
-    - name: Build
-      run: cmake --build build2 -j 2
-
-    - name: Python tests
-      run: cmake --build build2 --target pytest
-
-    - name: C++ tests
-      run: cmake --build build2 --target cpptest
-
-    # Third build - C++17 mode with unstable ABI
-    - name: Configure (unstable ABI)
-      run: >
-        cmake -S . -B build3
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=17
-        -DPYBIND11_INTERNALS_VERSION=10000000
-        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
-        ${{ matrix.args }}
-
-    - name: Build (unstable ABI)
-      run: cmake --build build3 -j 2
-
-    - name: Python tests (unstable ABI)
-      run: cmake --build build3 --target pytest
-
-    - name: Interface test
-      run: cmake --build build3 --target test_cmake_build
-
-    # This makes sure the setup_helpers module can build packages using
-    # setuptools
-    - name: Setuptools helpers test
-      run: pytest tests/extra_setuptools
diff --git a/.github/workflows/windows_clang.yml b/.github/workflows/windows_clang.yml
new file mode 100644
index 0000000000..703118eca3
--- /dev/null
+++ b/.github/workflows/windows_clang.yml
@@ -0,0 +1,77 @@
+name: WindowsClang
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  windows_clang:
+
+    strategy:
+      matrix:
+        os: [windows-latest]
+        python: ['3.10']
+
+    runs-on: "${{ matrix.os }}"
+
+    name: "🐍 ${{ matrix.python }} • ${{ matrix.os }} • clang-latest"
+
+    steps:
+      - name: Show env
+        run: env
+
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Set up Clang
+        uses: egor-tensin/setup-clang@v1
+
+      - name: Setup Python ${{ matrix.python }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Update CMake
+        uses: jwlawson/actions-setup-cmake@v1.13
+
+      - name: Install ninja-build tool
+        uses: seanmiddleditch/gha-setup-ninja@v3
+
+      - name: Run pip installs
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -r tests/requirements.txt
+
+      - name: Show Clang++ version
+        run: clang++ --version
+
+      - name: Show CMake version
+        run: cmake --version
+
+      # TODO: WERROR=ON
+      - name: Configure Clang
+        run: >
+          cmake -G Ninja -S . -B .
+          -DCMAKE_VERBOSE_MAKEFILE=ON
+          -DPYBIND11_WERROR=OFF
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+
+      - name: Build
+        run: cmake --build . -j 2
+
+      - name: Python tests
+        run: cmake --build . --target pytest -j 2
+
+      - name: C++ tests
+        run: cmake --build . --target cpptest -j 2
+
+      - name: Interface test
+        run: cmake --build . --target test_cmake_build -j 2
+
+      - name: Clean directory
+        run: git clean -fdx
diff --git a/docs/classes.rst b/docs/classes.rst
index c0c53135b8..5fbe08870e 100644
--- a/docs/classes.rst
+++ b/docs/classes.rst
@@ -58,6 +58,15 @@ interactive Python session demonstrating this example is shown below:
     Static member functions can be bound in the same way using
     :func:`class_::def_static`.
 
+.. note::
+
+    Binding C++ types in unnamed namespaces (also known as anonymous namespaces)
+    works reliably only with GCC and MSVC, but not with CLANG, or if libc++ is used.
+    See `#4319 <https://github.com/pybind/pybind11/pull/4319>`_ for background.
+    If portability is a concern, it is therefore not recommended to bind C++
+    types in unnamed namespaces. It will be safest to manually pick unique
+    namespace names.
+
 Keyword and default arguments
 =============================
 It is possible to specify keyword and default arguments using the syntax
diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 6fd61098c4..31e9287318 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -34,7 +34,7 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 4
+#    define PYBIND11_INTERNALS_VERSION 5
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -114,7 +114,7 @@ inline void tls_replace_value(PYBIND11_TLS_KEY_REF key, void *value) {
 // libstdc++, this doesn't happen: equality and the type_index hash are based on the type name,
 // which works.  If not under a known-good stl, provide our own name-based hash and equality
 // functions that use the type name.
-#if defined(__GLIBCXX__)
+#if !defined(_LIBCPP_VERSION)
 inline bool same_type(const std::type_info &lhs, const std::type_info &rhs) { return lhs == rhs; }
 using type_hash = std::hash<std::type_index>;
 using type_equal_to = std::equal_to<std::type_index>;
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 491f215cef..285f43b5b9 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -156,6 +156,8 @@ set(PYBIND11_TEST_FILES
     test_tagbased_polymorphic
     test_thread
     test_union
+    test_unnamed_namespace_a
+    test_unnamed_namespace_b
     test_virtual_functions)
 
 # Invoking cmake with something like:
diff --git a/tests/test_unnamed_namespace_a.cpp b/tests/test_unnamed_namespace_a.cpp
new file mode 100644
index 0000000000..e8704edca0
--- /dev/null
+++ b/tests/test_unnamed_namespace_a.cpp
@@ -0,0 +1,31 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_a, m) {
+    if (py::detail::get_type_info(typeid(any_struct)) == nullptr) {
+        py::class_<any_struct>(m, "unnamed_namespace_a_any_struct");
+    } else {
+        m.attr("unnamed_namespace_a_any_struct") = py::none();
+    }
+    m.attr("defined_WIN32_or__WIN32") =
+#if defined(WIN32) || defined(_WIN32)
+        true;
+#else
+        false;
+#endif
+    m.attr("defined___clang__") =
+#if defined(__clang__)
+        true;
+#else
+        false;
+#endif
+    m.attr("defined__LIBCPP_VERSION") =
+#if defined(_LIBCPP_VERSION)
+        true;
+#else
+        false;
+#endif
+}
diff --git a/tests/test_unnamed_namespace_a.py b/tests/test_unnamed_namespace_a.py
new file mode 100644
index 0000000000..be4f4bfe9b
--- /dev/null
+++ b/tests/test_unnamed_namespace_a.py
@@ -0,0 +1,30 @@
+import pytest
+
+from pybind11_tests import unnamed_namespace_a as m
+from pybind11_tests import unnamed_namespace_b as mb
+
+XFAIL_CONDITION = "not m.defined_WIN32_or__WIN32 and (m.defined___clang__ or m.defined__LIBCPP_VERSION)"
+XFAIL_REASON = "Known issues: https://github.com/pybind/pybind11/pull/4319"
+
+
+@pytest.mark.xfail(XFAIL_CONDITION, reason=XFAIL_REASON, strict=False)
+@pytest.mark.parametrize(
+    "any_struct", (m.unnamed_namespace_a_any_struct, mb.unnamed_namespace_b_any_struct)
+)
+def test_have_class_any_struct(any_struct):
+    assert any_struct is not None
+
+
+def test_have_at_least_one_class_any_struct():
+    assert (
+        m.unnamed_namespace_a_any_struct is not None
+        or mb.unnamed_namespace_b_any_struct is not None
+    )
+
+
+@pytest.mark.xfail(XFAIL_CONDITION, reason=XFAIL_REASON, strict=True)
+def test_have_both_class_any_struct():
+    assert (
+        m.unnamed_namespace_a_any_struct is not None
+        and mb.unnamed_namespace_b_any_struct is not None
+    )
diff --git a/tests/test_unnamed_namespace_b.cpp b/tests/test_unnamed_namespace_b.cpp
new file mode 100644
index 0000000000..f97757a7de
--- /dev/null
+++ b/tests/test_unnamed_namespace_b.cpp
@@ -0,0 +1,13 @@
+#include "pybind11_tests.h"
+
+namespace {
+struct any_struct {};
+} // namespace
+
+TEST_SUBMODULE(unnamed_namespace_b, m) {
+    if (py::detail::get_type_info(typeid(any_struct)) == nullptr) {
+        py::class_<any_struct>(m, "unnamed_namespace_b_any_struct");
+    } else {
+        m.attr("unnamed_namespace_b_any_struct") = py::none();
+    }
+}
diff --git a/tests/test_unnamed_namespace_b.py b/tests/test_unnamed_namespace_b.py
new file mode 100644
index 0000000000..4bcaa7a6c5
--- /dev/null
+++ b/tests/test_unnamed_namespace_b.py
@@ -0,0 +1,5 @@
+from pybind11_tests import unnamed_namespace_b as m
+
+
+def test_have_attr_any_struct():
+    assert hasattr(m, "unnamed_namespace_b_any_struct")