diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 041e0dfb3..967587643 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,12 +7,13 @@ on:
     branches:
       - master
       - stable
+      - smart_holder
       - v*
 
 permissions: read-all
 
 concurrency:
-  group: test-${{ github.ref }}
+  group: test-sh-avl${{ github.ref }}
   cancel-in-progress: true
 
 env:
diff --git a/.github/workflows/ci_sh_def.yml b/.github/workflows/ci_sh_def.yml
new file mode 100644
index 000000000..fd11911db
--- /dev/null
+++ b/.github/workflows/ci_sh_def.yml
@@ -0,0 +1,1270 @@
+# PLEASE KEEP THIS GROUP OF FILES IN SYNC AT ALL TIMES:
+#     ci.yml
+#     ci_sh_def.yml
+#     ci_sh_def.yml.patch
+# To import changes made to ci.yml *** ESPECIALLY AFTER `git merge master` ***:
+#     patch -i ci_sh_def.yml.patch -o ci_sh_def.yml
+# To update the patch file after making changes to ci_sh.yml:
+#     diff -u ci.yml ci_sh_def.yml > ci_sh_def.yml.patch
+#     git commit -a -m 'Tracking ci.yml changes from master.'
+#
+# Thanks a lot to @rhaschke for PR #2930!
+
+name: "CI-SH-DEF"
+
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches:
+      - master
+      - stable
+      - smart_holder
+      - v*
+
+permissions: read-all
+
+concurrency:
+  group: test-sh-def-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  PIP_BREAK_SYSTEM_PACKAGES: 1
+  PIP_ONLY_BINARY: numpy
+  FORCE_COLOR: 3
+  PYTEST_TIMEOUT: 300
+  # For cmake:
+  VERBOSE: 1
+
+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-20.04, windows-2022, macos-13]
+        python:
+        - '3.8'
+        - '3.9'
+        - '3.12'
+        - '3.13'
+        - 'pypy-3.8'
+        - 'pypy-3.9'
+        - 'pypy-3.10'
+
+        # 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-20.04
+            python: '3.8'
+            args: >
+              -DPYBIND11_FINDPYTHON=ON
+              -DCMAKE_CXX_FLAGS="-D_=1"
+            exercise_D_: 1
+          - runs-on: ubuntu-20.04
+            python: 'pypy-3.8'
+            args: >
+              -DPYBIND11_FINDPYTHON=ON
+          - runs-on: windows-2019
+            python: '3.8'
+            args: >
+              -DPYBIND11_FINDPYTHON=ON
+          # Inject a couple Windows 2019 runs
+          - runs-on: windows-2019
+            python: '3.9'
+          # Extra ubuntu latest job
+          - runs-on: ubuntu-latest
+            python: '3.11'
+
+
+    name: "🐍 ${{ matrix.python }} • ${{ matrix.runs-on }} • x64 ${{ matrix.args }}"
+    runs-on: ${{ matrix.runs-on }}
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup Python ${{ matrix.python }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python }}
+        allow-prereleases: true
+
+    - name: Setup Boost (Linux)
+      # Can't use boost + define _
+      if: runner.os == 'Linux' && matrix.exercise_D_ != 1
+      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@v2.0
+
+    - name: Cache wheels
+      if: runner.os == 'macOS'
+      uses: actions/cache@v4
+      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
+    # (same for PYBIND11_NUMPY_1_ONLY, but requires a NumPy 1.x at runtime).
+    - name: Configure C++11 ${{ matrix.args }}
+      run: >
+        cmake -S . -B .
+        -DPYBIND11_WERROR=ON
+        -DPYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION=ON
+        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
+        -DPYBIND11_NUMPY_1_ONLY=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=11
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+        ${{ 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
+      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.
+    # (same for PYBIND11_NUMPY_1_ONLY, but requires a NumPy 1.x at runtime).
+    - name: Configure C++17
+      run: >
+        cmake -S . -B build2
+        -DPYBIND11_WERROR=ON
+        -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+        -DPYBIND11_NUMPY_1_ONLY=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+        ${{ matrix.args }}
+
+    - 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
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+        -DPYBIND11_INTERNALS_VERSION=10000000
+        ${{ 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: |
+        pip install setuptools
+        pytest tests/extra_setuptools
+      if: "!(matrix.runs-on == 'windows-2022')"
+
+  manylinux:
+    name: Manylinux on 🐍 3.13t • GIL
+    runs-on: ubuntu-latest
+    timeout-minutes: 40
+    container: quay.io/pypa/musllinux_1_2_x86_64:latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Prepare venv
+        run: python3.13t -m venv .venv
+
+      - name: Install Python deps
+        run: .venv/bin/pip install -r tests/requirements.txt
+
+      - name: Configure C++11
+        run: >
+          cmake -S. -Bbuild
+          -DPYBIND11_WERROR=ON
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DPython_ROOT_DIR=.venv
+          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+
+      - name: Build C++11
+        run: cmake --build build -j2
+
+      - name: Python tests C++11
+        run: cmake --build build --target pytest -j2
+
+  deadsnakes:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+        # TODO: Fails on 3.10, investigate
+        # JOB DISABLED (NEEDS WORK): https://github.com/pybind/pybind11/issues/4889
+        # - 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-20.04
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup Python ${{ matrix.python-version }} (deadsnakes)
+      uses: deadsnakes/action@v3.1.0
+      with:
+        python-version: ${{ matrix.python-version }}
+        debug: ${{ matrix.python-debug }}
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v2.0
+
+    - name: Valgrind cache
+      if: matrix.valgrind
+      uses: actions/cache@v4
+      id: cache-valgrind
+      with:
+        path: valgrind
+        key: 3.16.1 # Valgrind version
+
+    - name: Compile Valgrind
+      if: matrix.valgrind && steps.cache-valgrind.outputs.cache-hit != 'true'
+      run: |
+        VALGRIND_VERSION=3.16.1
+        curl https://sourceware.org/pub/valgrind/valgrind-$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
+      run: >
+        cmake -S . -B build
+        -DCMAKE_BUILD_TYPE=Debug
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+
+    - 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
+        container_suffix:
+          - ""
+        include:
+          - clang: 5
+            std: 14
+          - clang: 10
+            std: 17
+          - clang: 11
+            std: 20
+          - clang: 12
+            std: 20
+          - clang: 13
+            std: 20
+          - clang: 14
+            std: 20
+          - clang: 15
+            std: 20
+            container_suffix: "-bullseye"
+          - clang: 16
+            std: 20
+            container_suffix: "-bullseye"
+
+    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64"
+    container: "silkeh/clang:${{ matrix.clang }}${{ matrix.container_suffix }}"
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - 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=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -DPYTHON_EXECUTABLE=$(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 12.2 • Ubuntu 22.04"
+    container: nvidia/cuda:12.2.0-devel-ubuntu22.04
+
+    steps:
+    - uses: actions/checkout@v4
+
+    # 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 -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+
+    - 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@v4
+#
+#    - 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=$(python3 -c "import sys; print(sys.executable)")
+#
+#    - name: Build
+#      run: cmake --build build -j 2 --verbose
+#
+#    - name: Python tests
+#      run: cmake --build build --target pytest
+#
+#    - name: C++ tests
+#      run: cmake --build build --target cpptest
+#
+#    - name: Interface test
+#      run: cmake --build build --target test_cmake_build
+
+
+  # Testing on Ubuntu + NVHPC (previous PGI) compilers, which seems to require more workarounds
+  ubuntu-nvhpc7:
+    runs-on: ubuntu-20.04
+    name: "🐍 3 • NVHPC 23.5 • C++17 • x64"
+
+    env:
+      # tzdata will try to ask for the timezone, so set the DEBIAN_FRONTEND
+      DEBIAN_FRONTEND: 'noninteractive'
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Add NVHPC Repo
+      run: |
+        echo 'deb [trusted=yes] https://developer.download.nvidia.com/hpc-sdk/ubuntu/amd64 /' | \
+          sudo tee /etc/apt/sources.list.d/nvhpc.list
+
+    - name: Install 🐍 3 & NVHPC
+      run: |
+        sudo apt-get update -y && \
+        sudo apt-get install -y cmake environment-modules git python3-dev python3-pip python3-numpy && \
+        sudo apt-get install -y --no-install-recommends nvhpc-23-5 && \
+        sudo rm -rf /var/lib/apt/lists/*
+        python3 -m pip install --upgrade pip
+        python3 -m pip install --upgrade pytest
+
+    # On some systems, you many need further workarounds:
+    # https://github.com/pybind/pybind11/pull/2475
+    - name: Configure
+      shell: bash
+      run: |
+        source /etc/profile.d/modules.sh
+        module load /opt/nvidia/hpc_sdk/modulefiles/nvhpc/23.5
+        cmake -S . -B build -DDOWNLOAD_CATCH=ON \
+                            -DCMAKE_CXX_STANDARD=17 \
+                            -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)") \
+                            -DCMAKE_CXX_FLAGS="-Wc,--pending_instantiations=0 -DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT" \
+                            -DPYBIND11_TEST_FILTER="test_smart_ptr.cpp"
+
+    - 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 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: 9, std: 20 }
+          - { gcc: 10, std: 17 }
+          - { gcc: 10, std: 20 }
+          - { gcc: 11, std: 20 }
+          - { gcc: 12, std: 20 }
+          - { gcc: 13, std: 20 }
+
+    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }}• x64"
+    container: "gcc:${{ matrix.gcc }}"
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Add Python 3
+      run: apt-get update; apt-get install -y python3-dev python3-numpy python3-pytest python3-pip libeigen3-dev
+
+    - name: Update pip
+      run: python3 -m pip install --upgrade pip
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v2.0
+
+    - name: Configure
+      shell: bash
+      run: >
+        cmake -S . -B build
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+
+    - name: Build
+      run: cmake --build build -j 2
+
+    - name: Python tests
+      run: cmake --build build --target pytest
+
+    - name: C++ tests
+      run: cmake --build build --target cpptest
+
+    - name: Interface test
+      run: cmake --build build --target test_cmake_build
+
+    - name: Configure - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      shell: bash
+      run: >
+        cmake -S . -B build_partial
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+    - name: Build - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      run: cmake --build build_partial -j 2
+
+    - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      if: matrix.gcc == '12'
+      run: cmake --build build_partial --target pytest
+
+  # Testing on ICC using the oneAPI apt repo
+  icc:
+    runs-on: ubuntu-20.04
+
+    name: "🐍 3 • ICC latest • x64"
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Add apt repo
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y wget build-essential pkg-config cmake ca-certificates gnupg
+        wget https://apt.repos.intel.com/intel-gpg-keys/GPG-PUB-KEY-INTEL-SW-PRODUCTS-2023.PUB
+        sudo apt-key add GPG-PUB-KEY-INTEL-SW-PRODUCTS-2023.PUB
+        echo "deb https://apt.repos.intel.com/oneapi all main" | sudo tee /etc/apt/sources.list.d/oneAPI.list
+
+    - name: Add ICC & Python 3
+      run: sudo apt-get update; sudo apt-get install -y intel-oneapi-compiler-dpcpp-cpp-and-cpp-classic cmake python3-dev python3-numpy python3-pytest python3-pip
+
+    - name: Update pip
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        python3 -m pip install --upgrade pip
+
+    - name: Install dependencies
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        python3 -m pip install -r tests/requirements.txt
+
+    - name: Configure C++11
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        cmake -S . -B build-11     \
+        -DPYBIND11_WERROR=ON    \
+        -DDOWNLOAD_CATCH=ON     \
+        -DDOWNLOAD_EIGEN=OFF    \
+        -DCMAKE_CXX_STANDARD=11             \
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"  \
+        -DCMAKE_CXX_COMPILER=$(which icpc)  \
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+
+    - name: Build C++11
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        cmake --build build-11 -j 2 -v
+
+    - name: Python tests C++11
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        sudo service apport stop
+        cmake --build build-11 --target check
+
+    - name: C++ tests C++11
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        cmake --build build-11 --target cpptest
+
+    - name: Interface test C++11
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        cmake --build build-11 --target test_cmake_build
+
+    - name: Configure C++17
+      run: |
+        set +e; source /opt/intel/oneapi/setvars.sh; set -e
+        cmake -S . -B build-17     \
+        -DPYBIND11_WERROR=ON    \
+        -DDOWNLOAD_CATCH=ON     \
+        -DDOWNLOAD_EIGEN=OFF    \
+        -DCMAKE_CXX_STANDARD=17             \
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"  \
+        -DCMAKE_CXX_COMPILER=$(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).
+  centos:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        container:
+          - "almalinux:8"
+          - "almalinux:9"
+
+    name: "🐍 3 • ${{ matrix.container }} • x64"
+    container: "${{ matrix.container }}"
+
+    steps:
+    - name: Latest actions/checkout
+      uses: actions/checkout@v4
+
+    - name: Add Python 3.8
+      if: matrix.container == 'almalinux:8'
+      run: dnf update -y && dnf install -y python38-devel gcc-c++ make git
+
+    - name: Add Python 3 (default)
+      if: matrix.container != 'almalinux:8'
+      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: Ensure NumPy 2 is used (required Python >= 3.9)
+      if: matrix.container == 'almalinux:9'
+      run: |
+        python3 -m pip install 'numpy>=2.0.0b1' 'scipy>=1.13.0rc1'
+
+    - 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
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+
+    - name: Build
+      run: cmake --build build -j 2
+
+    - name: Python tests
+      run: cmake --build build --target pytest
+
+    - name: C++ tests
+      run: cmake --build build --target cpptest
+
+    - name: Interface test
+      run: cmake --build build --target test_cmake_build
+
+
+  # This tests an "install" with the CMake tools
+  install-classic:
+    name: "🐍 3.7 • Debian • x86 •  Install"
+    runs-on: ubuntu-latest
+    container: i386/debian:buster
+
+    steps:
+    - uses: actions/checkout@v1  # v1 is required to run inside docker
+
+    - name: Install requirements
+      run: |
+        apt-get update
+        apt-get install -y git make cmake g++ libeigen3-dev python3-dev python3-pip
+        pip3 install "pytest==6.*"
+
+    - name: Configure for install
+      run: >
+        cmake .
+        -DPYBIND11_INSTALL=1 -DPYBIND11_TEST=0
+        -DPYTHON_EXECUTABLE=$(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
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+      working-directory: /build-tests
+
+    - name: Python tests
+      run: make pytest -j 2
+      working-directory: /build-tests
+
+
+  # This verifies that the documentation is not horribly broken, and does a
+  # basic validation check on the SDist.
+  doxygen:
+    name: "Documentation build test"
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - uses: actions/setup-python@v5
+      with:
+        python-version: "3.x"
+
+    - name: Install Doxygen
+      run: sudo apt-get install -y doxygen librsvg2-bin # Changed to rsvg-convert in 20.04
+
+    - name: Build docs
+      run: pipx run nox -s docs
+
+    - name: Make SDist
+      run: pipx run nox -s build -- --sdist
+
+    - run: git status --ignored
+
+    - name: Check local include dir
+      run: >
+        ls pybind11;
+        python3 -c "import pybind11, pathlib; assert (a := pybind11.get_include()) == (b := str(pathlib.Path('include').resolve())), f'{a} != {b}'"
+
+    - name: Compare Dists (headers only)
+      working-directory: include
+      run: |
+        python3 -m pip install --user -U ../dist/*.tar.gz
+        installed=$(python3 -c "import pybind11; print(pybind11.get_include() + '/pybind11')")
+        diff -rq $installed ./pybind11
+
+  win32:
+    strategy:
+      fail-fast: false
+      matrix:
+        python:
+        - '3.7'
+        - '3.8'
+        - '3.9'
+        - '3.10'
+        - '3.11'
+        - '3.12'
+
+        include:
+          - python: '3.12'
+            args: -DCMAKE_CXX_STANDARD=20
+          - python: '3.11'
+            args: -DCMAKE_CXX_STANDARD=20
+          - python: '3.10'
+            args: -DCMAKE_CXX_STANDARD=20
+          - 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@v4
+
+    - name: Setup Python ${{ matrix.python }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python }}
+        architecture: x86
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v2.0
+
+    - name: Prepare MSVC
+      uses: ilammy/msvc-dev-cmd@v1.13.0
+      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
+        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        ${{ 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@v4
+
+    - name: Setup Python ${{ matrix.python }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python }}
+        architecture: x86
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v2.0
+
+    - name: Prepare MSVC
+      uses: ilammy/msvc-dev-cmd@v1.13.0
+      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
+        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        ${{ 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@v4
+
+    - name: Setup Python ${{ matrix.python }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python }}
+
+    - name: Prepare env
+      # Ensure use of NumPy 2 (via NumPy nightlies but can be changed soon)
+      run: |
+        python3 -m pip install -r tests/requirements.txt
+        python3 -m pip install 'numpy>=2.0.0b1' 'scipy>=1.13.0rc1'
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v2.0
+
+    - name: Configure C++20
+      run: >
+        cmake -S . -B build
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=20
+        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+
+    - 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
+
+    - name: Configure C++20 - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: >
+        cmake -S . -B build_partial
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=20
+        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+    - name: Build C++20 - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: cmake --build build_partial -j 2
+
+    - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+      run: cmake --build build_partial --target pytest
+
+  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-${{matrix.env}}-cmake
+          mingw-w64-${{matrix.env}}-make
+          mingw-w64-${{matrix.env}}-python-pytest
+          mingw-w64-${{matrix.env}}-boost
+          mingw-w64-${{matrix.env}}-catch
+
+    - uses: msys2/setup-msys2@v2
+      if: matrix.sys == 'mingw64'
+      with:
+        msystem: ${{matrix.sys}}
+        install: >-
+          git
+          mingw-w64-${{matrix.env}}-python-scipy
+          mingw-w64-${{matrix.env}}-eigen3
+
+    - uses: actions/checkout@v4
+
+    - name: Configure C++11
+      # LTO leads to many undefined reference like
+      # `pybind11::detail::function_call::function_call(pybind11::detail::function_call&&)
+      run: >-
+        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -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 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -S . -B build2
+
+    - name: Build C++14
+      run: cmake --build build2 -j 2
+
+    - name: Python tests C++14
+      run: cmake --build build2 --target pytest -j 2
+
+    - name: C++14 tests
+      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target cpptest -j 2
+
+    - name: Interface test C++14
+      run: PYTHONHOME=/${{matrix.sys}} PYTHONPATH=/${{matrix.sys}} cmake --build build2 --target test_cmake_build
+
+    - name: Clean directory
+      run: git clean -fdx
+
+    - name: Configure C++17
+      run: >-
+        cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
+        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+        -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
+
+  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@v4
+
+      - name: Set up Clang
+        uses: egor-tensin/setup-clang@v1
+
+      - name: Setup Python ${{ matrix.python }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python }}
+
+      - name: Update CMake
+        uses: jwlawson/actions-setup-cmake@v2.0
+
+      - name: Install ninja-build tool
+        uses: seanmiddleditch/gha-setup-ninja@v5
+
+      - 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 .
+          -DPYBIND11_WERROR=OFF
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+
+      - 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
+
+  macos_brew_install_llvm:
+    name: "macos-13 • brew install llvm"
+    runs-on: macos-13
+
+    env:
+      # https://apple.stackexchange.com/questions/227026/how-to-install-recent-clang-with-homebrew
+      LDFLAGS: '-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib'
+
+    steps:
+      - name: Update PATH
+        run: echo "/usr/local/opt/llvm/bin" >> $GITHUB_PATH
+
+      - name: Show env
+        run: env
+
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Show Clang++ version before brew install llvm
+        run: clang++ --version
+
+      - name: brew install llvm
+        run: brew install llvm
+
+      - name: Show Clang++ version after brew install llvm
+        run: clang++ --version
+
+      - name: Update CMake
+        uses: jwlawson/actions-setup-cmake@v2.0
+
+      - name: Run pip installs
+        run: |
+          python3 -m pip install --upgrade pip
+          python3 -m pip install -r tests/requirements.txt
+          python3 -m pip install numpy
+          python3 -m pip install scipy
+
+      - name: Show CMake version
+        run: cmake --version
+
+      - name: CMake Configure
+        run: >
+          cmake -S . -B .
+          -DPYBIND11_WERROR=ON
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+          -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+
+      - 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: CMake Configure - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: >
+          cmake -S . -B build_partial
+          -DPYBIND11_WERROR=ON
+          -DPYBIND11_SIMPLE_GIL_MANAGEMENT=OFF
+          -DDOWNLOAD_CATCH=ON
+          -DDOWNLOAD_EIGEN=ON
+          -DCMAKE_CXX_COMPILER=clang++
+          -DCMAKE_CXX_STANDARD=17
+          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+          -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+          "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+
+      - name: Build - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: cmake --build build_partial -j 2
+
+      - name: Python tests - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+        run: cmake --build build_partial --target pytest -j 2
+
+      - name: Clean directory
+        run: git clean -fdx
diff --git a/.github/workflows/ci_sh_def.yml.patch b/.github/workflows/ci_sh_def.yml.patch
new file mode 100644
index 000000000..e6f5e57f6
--- /dev/null
+++ b/.github/workflows/ci_sh_def.yml.patch
@@ -0,0 +1,223 @@
+--- ci.yml	2024-07-01 20:26:45.034547517 -0700
++++ ci_sh_def.yml	2024-07-01 20:27:32.110506039 -0700
+@@ -1,4 +1,16 @@
+-name: CI
++# PLEASE KEEP THIS GROUP OF FILES IN SYNC AT ALL TIMES:
++#     ci.yml
++#     ci_sh_def.yml
++#     ci_sh_def.yml.patch
++# To import changes made to ci.yml *** ESPECIALLY AFTER `git merge master` ***:
++#     patch -i ci_sh_def.yml.patch -o ci_sh_def.yml
++# To update the patch file after making changes to ci_sh.yml:
++#     diff -u ci.yml ci_sh_def.yml > ci_sh_def.yml.patch
++#     git commit -a -m 'Tracking ci.yml changes from master.'
++#
++# Thanks a lot to @rhaschke for PR #2930!
++
++name: "CI-SH-DEF"
+ 
+ on:
+   workflow_dispatch:
+@@ -13,7 +25,7 @@
+ permissions: read-all
+ 
+ concurrency:
+-  group: test-sh-avl${{ github.ref }}
++  group: test-sh-def-${{ github.ref }}
+   cancel-in-progress: true
+ 
+ env:
+@@ -126,6 +138,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=11
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+         ${{ matrix.args }}
+ 
+     - name: Build C++11
+@@ -155,6 +168,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=17
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+         ${{ matrix.args }}
+ 
+     - name: Build
+@@ -174,6 +188,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=17
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT ${{runner.os == 'Windows' && '/GR /EHsc' || ''}}"
+         -DPYBIND11_INTERNALS_VERSION=10000000
+         ${{ matrix.args }}
+ 
+@@ -217,6 +232,7 @@
+           -DDOWNLOAD_CATCH=ON
+           -DDOWNLOAD_EIGEN=ON
+           -DPython_ROOT_DIR=.venv
++          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+ 
+       - name: Build C++11
+         run: cmake --build build -j2
+@@ -290,6 +306,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=17
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+ 
+     - name: Build
+       run: cmake --build build -j 2
+@@ -358,6 +375,7 @@
+         -DPYBIND11_WERROR=ON
+         -DDOWNLOAD_CATCH=ON
+         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+     - name: Build
+@@ -387,7 +405,7 @@
+       run: apt-get update && DEBIAN_FRONTEND="noninteractive" apt-get install -y cmake git python3-dev python3-pytest python3-numpy
+ 
+     - name: Configure
+-      run: cmake -S . -B build -DPYBIND11_CUDA_TESTS=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
++      run: cmake -S . -B build -DPYBIND11_CUDA_TESTS=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+ 
+     - name: Build
+       run: cmake --build build -j2 --verbose
+@@ -475,7 +493,7 @@
+         cmake -S . -B build -DDOWNLOAD_CATCH=ON \
+                             -DCMAKE_CXX_STANDARD=17 \
+                             -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)") \
+-                            -DCMAKE_CXX_FLAGS="-Wc,--pending_instantiations=0" \
++                            -DCMAKE_CXX_FLAGS="-Wc,--pending_instantiations=0 -DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT" \
+                             -DPYBIND11_TEST_FILTER="test_smart_ptr.cpp"
+ 
+     - name: Build
+@@ -531,6 +549,7 @@
+         -DPYBIND11_WERROR=ON
+         -DDOWNLOAD_CATCH=ON
+         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+     - name: Build
+@@ -553,6 +572,7 @@
+         -DPYBIND11_WERROR=ON
+         -DDOWNLOAD_CATCH=ON
+         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+         "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+ 
+@@ -602,6 +622,7 @@
+         -DDOWNLOAD_CATCH=ON     \
+         -DDOWNLOAD_EIGEN=OFF    \
+         -DCMAKE_CXX_STANDARD=11             \
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"  \
+         -DCMAKE_CXX_COMPILER=$(which icpc)  \
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+@@ -634,6 +655,7 @@
+         -DDOWNLOAD_CATCH=ON     \
+         -DDOWNLOAD_EIGEN=OFF    \
+         -DCMAKE_CXX_STANDARD=17             \
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"  \
+         -DCMAKE_CXX_COMPILER=$(which icpc)  \
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+@@ -705,6 +727,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=11
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+     - name: Build
+@@ -755,6 +778,7 @@
+         cmake ../pybind11-tests
+         -DDOWNLOAD_CATCH=ON
+         -DPYBIND11_WERROR=ON
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+       working-directory: /build-tests
+ 
+@@ -858,6 +882,7 @@
+         -DPYBIND11_WERROR=ON
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
++        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         ${{ matrix.args }}
+     - name: Build C++11
+       run: cmake --build build -j 2
+@@ -912,6 +937,7 @@
+         -DPYBIND11_WERROR=ON
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
++        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         ${{ matrix.args }}
+     - name: Build C++11
+       run: cmake --build build --config Debug -j 2
+@@ -954,6 +980,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=20
++        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+ 
+     - name: Build C++20
+       run: cmake --build build -j 2
+@@ -974,6 +1001,7 @@
+         -DDOWNLOAD_CATCH=ON
+         -DDOWNLOAD_EIGEN=ON
+         -DCMAKE_CXX_STANDARD=20
++        -DCMAKE_CXX_FLAGS="/GR /EHsc /DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+ 
+     - name: Build C++20 - Exercise cmake -DPYBIND11_TEST_OVERRIDE
+@@ -1026,6 +1054,7 @@
+       run: >-
+         cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+         -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -S . -B build
+ 
+     - name: Build C++11
+@@ -1047,6 +1076,7 @@
+       run: >-
+         cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+         -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -S . -B build2
+ 
+     - name: Build C++14
+@@ -1068,6 +1098,7 @@
+       run: >-
+         cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON
+         -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
++        -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+         -S . -B build3
+ 
+     - name: Build C++17
+@@ -1135,6 +1166,7 @@
+           -DDOWNLOAD_EIGEN=ON
+           -DCMAKE_CXX_COMPILER=clang++
+           -DCMAKE_CXX_STANDARD=17
++          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+ 
+       - name: Build
+         run: cmake --build . -j 2
+@@ -1200,6 +1232,7 @@
+           -DDOWNLOAD_EIGEN=ON
+           -DCMAKE_CXX_COMPILER=clang++
+           -DCMAKE_CXX_STANDARD=17
++          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+           -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+ 
+       - name: Build
+@@ -1223,6 +1256,7 @@
+           -DDOWNLOAD_EIGEN=ON
+           -DCMAKE_CXX_COMPILER=clang++
+           -DCMAKE_CXX_STANDARD=17
++          -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT"
+           -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
+           "-DPYBIND11_TEST_OVERRIDE=test_call_policies.cpp;test_gil_scoped.cpp;test_thread.cpp"
+ 
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index 0e55a0795..ac3900c3f 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -7,6 +7,7 @@ on:
     branches:
       - master
       - stable
+      - smart_holder
       - v*
 
 permissions:
diff --git a/.github/workflows/format.yml b/.github/workflows/format.yml
index 1eaa56e1c..85bcc3c6c 100644
--- a/.github/workflows/format.yml
+++ b/.github/workflows/format.yml
@@ -10,6 +10,7 @@ on:
     branches:
     - master
     - stable
+    - smart_holder
     - "v*"
 
 permissions:
diff --git a/.github/workflows/pip.yml b/.github/workflows/pip.yml
index c1acb8bb4..90a6d3d2c 100644
--- a/.github/workflows/pip.yml
+++ b/.github/workflows/pip.yml
@@ -7,6 +7,7 @@ on:
     branches:
     - master
     - stable
+    - smart_holder
     - v*
   release:
     types:
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 92469eb37..8961a7773 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -76,6 +76,7 @@ repos:
   - id: mixed-line-ending
   - id: requirements-txt-fixer
   - id: trailing-whitespace
+    exclude: \.patch?$
 
 # Also code format the docs
 - repo: https://github.com/adamchainz/blacken-docs
@@ -90,6 +91,7 @@ repos:
   rev: "v1.5.5"
   hooks:
   - id: remove-tabs
+    exclude: (^docs/.*|\.patch)?$
 
 # Avoid directional quotes
 - repo: https://github.com/sirosen/texthooks