diff --git a/.clang-tidy b/.clang-tidy
index d01ca352c..e82443c4c 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -25,11 +25,12 @@ modernize-replace-random-shuffle,
 modernize-shrink-to-fit,
 modernize-use-auto,
 modernize-use-bool-literals,
+modernize-use-default-member-init,
 modernize-use-equals-default,
 modernize-use-equals-delete,
-modernize-use-default-member-init,
-modernize-use-noexcept,
 modernize-use-emplace,
+modernize-use-noexcept,
+modernize-use-nullptr,
 modernize-use-override,
 modernize-use-using,
 *performance*,
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3eb4e7b42..f784764b1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -32,6 +32,7 @@ jobs:
         - '3.10'
         - '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
@@ -45,6 +46,10 @@ jobs:
             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: >
@@ -668,7 +673,7 @@ jobs:
 
 
   # This verifies that the documentation is not horribly broken, and does a
-  # basic sanity check on the SDist.
+  # basic validation check on the SDist.
   doxygen:
     name: "Documentation build test"
     runs-on: ubuntu-latest
@@ -756,55 +761,60 @@ jobs:
     - name: Python tests
       run: cmake --build build -t pytest
 
-  win32-msvc2017:
-    name: "🐍 ${{ matrix.python }} • MSVC 2017 • x64"
-    runs-on: windows-2016
+  win32-debug:
     strategy:
       fail-fast: false
       matrix:
         python:
-          - 3.6
-          - 3.7
-        std:
-          - 14
+        - 3.8
+        - 3.9
 
         include:
-          - python: 3.7
-            std: 17
-            args: >
-              -DCMAKE_CXX_FLAGS="/permissive- /EHsc /GR"
+          - 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@v2
 
-    - name: Setup 🐍 ${{ matrix.python }}
+    - name: Setup Python ${{ matrix.python }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python }}
+        architecture: x86
 
     - name: Update CMake
       uses: jwlawson/actions-setup-cmake@v1.12
 
+    - name: Prepare MSVC
+      uses: ilammy/msvc-dev-cmd@v1.10.0
+      with:
+        arch: x86
+
     - name: Prepare env
       run: |
         python -m pip install -r tests/requirements.txt
 
     # First build - C++11 mode and inplace
-    - name: Configure
+    - name: Configure ${{ matrix.args }}
       run: >
         cmake -S . -B build
-        -G "Visual Studio 15 2017" -A x64
+        -G "Visual Studio 16 2019" -A Win32
+        -DCMAKE_BUILD_TYPE=Debug
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
-        -DCMAKE_CXX_STANDARD=${{ matrix.std }}
         ${{ matrix.args }}
+    - name: Build C++11
+      run: cmake --build build --config Debug -j 2
 
-    - name: Build ${{ matrix.std }}
-      run: cmake --build build -j 2
+    - name: Python tests
+      run: cmake --build build --config Debug -t pytest
 
-    - name: Run all checks
-      run: cmake --build build -t check
 
   windows-2022:
     strategy:
diff --git a/.github/workflows/configure.yml b/.github/workflows/configure.yml
index 66ab0e3d7..55be5bd34 100644
--- a/.github/workflows/configure.yml
+++ b/.github/workflows/configure.yml
@@ -18,7 +18,7 @@ jobs:
       matrix:
         runs-on: [ubuntu-latest, macos-latest, windows-latest]
         arch: [x64]
-        cmake: ["3.21"]
+        cmake: ["3.23"]
 
         include:
         - runs-on: ubuntu-latest
@@ -29,12 +29,8 @@ jobs:
           arch: x64
           cmake: 3.7
 
-        - runs-on: windows-2016
-          arch: x86
-          cmake: 3.8
-
-        - runs-on: windows-2016
-          arch: x86
+        - 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 }}
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index e9a0e03f2..099f0dc26 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -15,7 +15,7 @@
 repos:
 # Standard hooks
 - repo: https://github.com/pre-commit/pre-commit-hooks
-  rev: "v4.1.0"
+  rev: "v4.2.0"
   hooks:
   - id: check-added-large-files
   - id: check-case-conflict
@@ -32,7 +32,7 @@ repos:
 
 # Upgrade old Python syntax
 - repo: https://github.com/asottile/pyupgrade
-  rev: "v2.31.0"
+  rev: "v2.32.0"
   hooks:
   - id: pyupgrade
     args: [--py36-plus]
@@ -45,7 +45,7 @@ repos:
 
 # Black, the code formatter, natively supports pre-commit
 - repo: https://github.com/psf/black
-  rev: "22.1.0" # Keep in sync with blacken-docs
+  rev: "22.3.0" # Keep in sync with blacken-docs
   hooks:
   - id: black
 
@@ -55,7 +55,7 @@ repos:
   hooks:
   - id: blacken-docs
     additional_dependencies:
-    - black==22.1.0 # keep in sync with black hook
+    - black==22.3.0 # keep in sync with black hook
 
 # Changes tabs to spaces
 - repo: https://github.com/Lucas-C/pre-commit-hooks
@@ -64,16 +64,17 @@ repos:
   - id: remove-tabs
 
 - repo: https://github.com/sirosen/texthooks
-  rev: "0.2.2"
+  rev: "0.3.1"
   hooks:
   - id: fix-ligatures
   - id: fix-smartquotes
 
 # Autoremoves unused imports
 - repo: https://github.com/hadialqattan/pycln
-  rev: "v1.2.4"
+  rev: "v1.3.1"
   hooks:
   - id: pycln
+    stages: [manual]
 
 # Checking for common mistakes
 - repo: https://github.com/pre-commit/pygrep-hooks
@@ -106,7 +107,7 @@ repos:
 
 # PyLint has native support - not always usable, but works for us
 - repo: https://github.com/PyCQA/pylint
-  rev: "v2.12.2"
+  rev: "v2.13.5"
   hooks:
   - id: pylint
     files: ^pybind11
@@ -122,16 +123,16 @@ repos:
 
 # Check static types with mypy
 - repo: https://github.com/pre-commit/mirrors-mypy
-  rev: "v0.931"
+  rev: "v0.942"
   hooks:
   - id: mypy
-    args: [--show-error-codes]
+    args: []
     exclude: ^(tests|docs)/
     additional_dependencies: [nox, rich]
 
 # Checks the manifest for missing files (native support)
 - repo: https://github.com/mgedmin/check-manifest
-  rev: "0.47"
+  rev: "0.48"
   hooks:
   - id: check-manifest
     # This is a slow hook, so only run this if --hook-stage manual is passed
@@ -163,7 +164,7 @@ repos:
 
 # Clang format the codebase automatically
 - repo: https://github.com/pre-commit/mirrors-clang-format
-  rev: "v13.0.1"
+  rev: "v14.0.1"
   hooks:
   - id: clang-format
     types_or: [c++, c, cuda]
diff --git a/docs/advanced/pycpp/numpy.rst b/docs/advanced/pycpp/numpy.rst
index 8ad341004..b6ef019ed 100644
--- a/docs/advanced/pycpp/numpy.rst
+++ b/docs/advanced/pycpp/numpy.rst
@@ -87,7 +87,7 @@ buffer objects (e.g. a NumPy matrix).
             /* Request a buffer descriptor from Python */
             py::buffer_info info = b.request();
 
-            /* Some sanity checks ... */
+            /* Some basic validation checks ... */
             if (info.format != py::format_descriptor<Scalar>::format())
                 throw std::runtime_error("Incompatible format: expected a double array!");
 
diff --git a/docs/changelog.rst b/docs/changelog.rst
index 9d7693cdd..67ac79cdd 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -6,10 +6,129 @@ Changelog
 Starting with version 1.8.0, pybind11 releases use a `semantic versioning
 <http://semver.org>`_ policy.
 
+Changes will be added here periodically from the "Suggested changelog entry"
+block in pull request descriptions.
+
 IN DEVELOPMENT
 --------------
 
-Changes will be added here periodically.
+Removed support for Python 2.7, Python 3.5, and MSVC 2015. Support for MSVC
+2017 is limited due to availability of CI runners; we highly recommend MSVC
+2019 or 2022 be used.
+
+New features:
+
+* ``type_caster<std::monostate>`` was added. ``std::monostate`` is a tag type
+  that allows ``std::variant`` to act as an optional, or allows default
+  construction of a ``std::variant`` holding a non-default constructible type.
+  `#3818 <https://github.com/pybind/pybind11/pull/3818>`_
+
+* Support bytearray casting to string.
+  `#3707 <https://github.com/pybind/pybind11/pull/3707>`_
+
+Changes:
+
+* Python 2 support was removed completely.
+  `#3688 <https://github.com/pybind/pybind11/pull/3688>`_
+
+* The minimum version for MSVC is now 2017.
+  `#3722 <https://github.com/pybind/pybind11/pull/3722>`_
+
+* Improve exception handling in python ``str`` bindings.
+  `#3826 <https://github.com/pybind/pybind11/pull/3826>`_
+
+* The bindings for capsules now have more consistent exception handling.
+  `#3825 <https://github.com/pybind/pybind11/pull/3825>`_
+
+* Fix exception handling when ``pybind11::weakref()`` fails.
+  `#3739 <https://github.com/pybind/pybind11/pull/3739>`_
+
+
+Bug fixes:
+
+* ``PYBIND11_OBJECT_CVT`` and ``PYBIND11_OBJECT_CVT_DEFAULT`` macro can be used
+  to define classes in namespaces other than pybind11.
+  `#3797 <https://github.com/pybind/pybind11/pull/3797>`_
+
+Build system improvements:
+
+* Add MSVC builds in debug mode to CI.
+  `#3784 <https://github.com/pybind/pybind11/pull/3784>`_
+
+* MSVC 2022 C++20 coverage was added to GitHub Actions, including Eigen.
+  `#3732 <https://github.com/pybind/pybind11/pull/3732>`_,
+  `#3741 <https://github.com/pybind/pybind11/pull/3741>`_
+
+* Avoid ``setup.py <command>`` usage in internal tests.
+  `#3734 <https://github.com/pybind/pybind11/pull/3734>`_
+
+
+Backend and tidying up:
+
+* Remove idioms in code comments.  Use inclusive language.
+  `#3809 <https://github.com/pybind/pybind11/pull/3809>`_
+
+
+Version 2.9.2 (Mar 29, 2022)
+----------------------------
+
+Changes:
+
+* Enum now has an ``__index__`` method on Python <3.8 too.
+  `#3700 <https://github.com/pybind/pybind11/pull/3700>`_
+
+* Local internals are now cleared after finalizing the interpreter.
+  `#3744 <https://github.com/pybind/pybind11/pull/3744>`_
+
+Bug fixes:
+
+* Better support for Python 3.11 alphas.
+  `#3694 <https://github.com/pybind/pybind11/pull/3694>`_
+
+* ``PYBIND11_TYPE_CASTER`` now uses fully qualified symbols, so it can be used
+  outside of ``pybind11::detail``.
+  `#3758 <https://github.com/pybind/pybind11/pull/3758>`_
+
+* Some fixes for PyPy 3.9.
+  `#3768 <https://github.com/pybind/pybind11/pull/3768>`_
+
+* Fixed a potential memleak in PyPy in ``get_type_override``.
+  `#3774 <https://github.com/pybind/pybind11/pull/3774>`_
+
+* Fix usage of ``VISIBILITY_INLINES_HIDDEN``.
+  `#3721 <https://github.com/pybind/pybind11/pull/3721>`_
+
+
+Build system improvements:
+
+* Uses ``sysconfig`` module to determine installation locations on Python >=
+  3.10, instead of ``distutils`` which has been deprecated.
+  `#3764 <https://github.com/pybind/pybind11/pull/3764>`_
+
+* Support Catch 2.13.5+ (supporting GLIBC 2.34+).
+  `#3679 <https://github.com/pybind/pybind11/pull/3679>`_
+
+* Fix test failures with numpy 1.22 by ignoring whitespace when comparing
+  ``str()`` of dtypes.
+  `#3682 <https://github.com/pybind/pybind11/pull/3682>`_
+
+
+Backend and tidying up:
+
+* clang-tidy: added ``readability-qualified-auto``,
+  ``readability-braces-around-statements``,
+  ``cppcoreguidelines-prefer-member-initializer``,
+  ``clang-analyzer-optin.performance.Padding``,
+  ``cppcoreguidelines-pro-type-static-cast-downcast``, and
+  ``readability-inconsistent-declaration-parameter-name``.
+  `#3702 <https://github.com/pybind/pybind11/pull/3702>`_,
+  `#3699 <https://github.com/pybind/pybind11/pull/3699>`_,
+  `#3716 <https://github.com/pybind/pybind11/pull/3716>`_,
+  `#3709 <https://github.com/pybind/pybind11/pull/3709>`_
+
+* clang-format was added to the pre-commit actions, and the entire code base
+  automatically reformatted (after several iterations preparing for this leap).
+  `#3713 <https://github.com/pybind/pybind11/pull/3713>`_
 
 
 Version 2.9.1 (Feb 2, 2022)
diff --git a/docs/compiling.rst b/docs/compiling.rst
index 4ae9234e1..2b543be0b 100644
--- a/docs/compiling.rst
+++ b/docs/compiling.rst
@@ -505,7 +505,10 @@ You can use these targets to build complex applications. For example, the
     target_link_libraries(example PRIVATE pybind11::module pybind11::lto pybind11::windows_extras)
 
     pybind11_extension(example)
-    pybind11_strip(example)
+    if(NOT MSVC AND NOT ${CMAKE_BUILD_TYPE} MATCHES Debug|RelWithDebInfo)
+        # Strip unnecessary sections of the binary on Linux/macOS
+        pybind11_strip(example)
+    endif()
 
     set_target_properties(example PROPERTIES CXX_VISIBILITY_PRESET "hidden"
                                              CUDA_VISIBILITY_PRESET "hidden")
diff --git a/include/pybind11/cast.h b/include/pybind11/cast.h
index d45b49c52..e8128710e 100644
--- a/include/pybind11/cast.h
+++ b/include/pybind11/cast.h
@@ -514,7 +514,7 @@ struct type_caster<std::basic_string_view<CharT, Traits>,
 template <typename CharT>
 struct type_caster<CharT, enable_if_t<is_std_char_type<CharT>::value>> {
     using StringType = std::basic_string<CharT>;
-    using StringCaster = type_caster<StringType>;
+    using StringCaster = make_caster<StringType>;
     StringCaster str_caster;
     bool none = false;
     CharT one_char = 0;
@@ -1155,15 +1155,18 @@ enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_ref(object &&,
 // static_assert, even though if it's in dead code, so we provide a "trampoline" to pybind11::cast
 // that only does anything in cases where pybind11::cast is valid.
 template <typename T>
-enable_if_t<!cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&o) {
-    return pybind11::cast<T>(std::move(o));
-}
-template <typename T>
 enable_if_t<cast_is_temporary_value_reference<T>::value, T> cast_safe(object &&) {
     pybind11_fail("Internal error: cast_safe fallback invoked");
 }
-template <>
-inline void cast_safe<void>(object &&) {}
+template <typename T>
+enable_if_t<std::is_same<void, intrinsic_t<T>>::value, void> cast_safe(object &&) {}
+template <typename T>
+enable_if_t<detail::none_of<cast_is_temporary_value_reference<T>,
+                            std::is_same<void, intrinsic_t<T>>>::value,
+            T>
+cast_safe(object &&o) {
+    return pybind11::cast<T>(std::move(o));
+}
 
 PYBIND11_NAMESPACE_END(detail)
 
@@ -1243,8 +1246,8 @@ struct arg_v : arg {
 private:
     template <typename T>
     arg_v(arg &&base, T &&x, const char *descr = nullptr)
-        : arg(base), value(reinterpret_steal<object>(
-                         detail::make_caster<T>::cast(x, return_value_policy::automatic, {}))),
+        : arg(base), value(reinterpret_steal<object>(detail::make_caster<T>::cast(
+                         std::forward<T>(x), return_value_policy::automatic, {}))),
           descr(descr)
 #if !defined(NDEBUG)
           ,
@@ -1491,7 +1494,7 @@ private:
                                                         type_id<T>());
 #endif
         }
-        args_list.append(o);
+        args_list.append(std::move(o));
     }
 
     void process(list &args_list, detail::args_proxy ap) {
diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index ea09bb3fd..a130ebea0 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -992,6 +992,8 @@ constexpr const char
 struct error_scope {
     PyObject *type, *value, *trace;
     error_scope() { PyErr_Fetch(&type, &value, &trace); }
+    error_scope(const error_scope &) = delete;
+    error_scope &operator=(const error_scope &) = delete;
     ~error_scope() { PyErr_Restore(type, value, trace); }
 };
 
diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index bfdcf49e1..767b9044a 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -225,8 +225,8 @@ PYBIND11_NOINLINE detail::type_info *get_type_info(const std::type_index &tp,
     if (throw_if_missing) {
         std::string tname = tp.name();
         detail::clean_type_id(tname);
-        pybind11_fail("pybind11::detail::get_type_info: unable to find type info for \"" + tname
-                      + "\"");
+        pybind11_fail("pybind11::detail::get_type_info: unable to find type info for \""
+                      + std::move(tname) + '"');
     }
     return nullptr;
 }
diff --git a/include/pybind11/eigen.h b/include/pybind11/eigen.h
index 930c4f7fa..f658168de 100644
--- a/include/pybind11/eigen.h
+++ b/include/pybind11/eigen.h
@@ -668,7 +668,7 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
                                      Type::Flags &(Eigen::RowMajor | Eigen::ColMajor),
                                      StorageIndex>(shape[0].cast<Index>(),
                                                    shape[1].cast<Index>(),
-                                                   nnz,
+                                                   std::move(nnz),
                                                    outerIndices.mutable_data(),
                                                    innerIndices.mutable_data(),
                                                    values.mutable_data());
@@ -686,7 +686,8 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
         array outerIndices((rowMajor ? src.rows() : src.cols()) + 1, src.outerIndexPtr());
         array innerIndices(src.nonZeros(), src.innerIndexPtr());
 
-        return matrix_type(std::make_tuple(data, innerIndices, outerIndices),
+        return matrix_type(std::make_tuple(
+                               std::move(data), std::move(innerIndices), std::move(outerIndices)),
                            std::make_pair(src.rows(), src.cols()))
             .release();
     }
diff --git a/include/pybind11/eval.h b/include/pybind11/eval.h
index c157a0095..bd5f981f5 100644
--- a/include/pybind11/eval.h
+++ b/include/pybind11/eval.h
@@ -82,7 +82,7 @@ template <eval_mode mode = eval_expr, size_t N>
 object eval(const char (&s)[N], object global = globals(), object local = object()) {
     /* Support raw string literals by removing common leading whitespace */
     auto expr = (s[0] == '\n') ? str(module_::import("textwrap").attr("dedent")(s)) : str(s);
-    return eval<mode>(expr, global, local);
+    return eval<mode>(expr, std::move(global), std::move(local));
 }
 
 inline void exec(const str &expr, object global = globals(), object local = object()) {
@@ -91,7 +91,7 @@ inline void exec(const str &expr, object global = globals(), object local = obje
 
 template <size_t N>
 void exec(const char (&s)[N], object global = globals(), object local = object()) {
-    eval<eval_statements>(s, global, local);
+    eval<eval_statements>(s, std::move(global), std::move(local));
 }
 
 #if defined(PYPY_VERSION)
diff --git a/include/pybind11/numpy.h b/include/pybind11/numpy.h
index 5e68f6053..2713e13a8 100644
--- a/include/pybind11/numpy.h
+++ b/include/pybind11/numpy.h
@@ -263,7 +263,7 @@ private:
     static npy_api lookup() {
         module_ m = module_::import("numpy.core.multiarray");
         auto c = m.attr("_ARRAY_API");
-        void **api_ptr = (void **) PyCapsule_GetPointer(c.ptr(), NULL);
+        void **api_ptr = (void **) PyCapsule_GetPointer(c.ptr(), nullptr);
         npy_api api;
 #define DECL_NPY_API(Func) api.Func##_ = (decltype(api.Func##_)) api_ptr[API_##Func];
         DECL_NPY_API(PyArray_GetNDArrayCFeatureVersion);
@@ -540,18 +540,18 @@ public:
     PYBIND11_OBJECT_DEFAULT(dtype, object, detail::npy_api::get().PyArrayDescr_Check_);
 
     explicit dtype(const buffer_info &info) {
-        dtype descr(_dtype_from_pep3118()(PYBIND11_STR_TYPE(info.format)));
+        dtype descr(_dtype_from_pep3118()(pybind11::str(info.format)));
         // If info.itemsize == 0, use the value calculated from the format string
         m_ptr = descr.strip_padding(info.itemsize != 0 ? info.itemsize : descr.itemsize())
                     .release()
                     .ptr();
     }
 
-    explicit dtype(const std::string &format) {
-        m_ptr = from_args(pybind11::str(format)).release().ptr();
-    }
+    explicit dtype(const pybind11::str &format) : dtype(from_args(format)) {}
 
-    explicit dtype(const char *format) : dtype(std::string(format)) {}
+    explicit dtype(const std::string &format) : dtype(pybind11::str(format)) {}
+
+    explicit dtype(const char *format) : dtype(pybind11::str(format)) {}
 
     dtype(list names, list formats, list offsets, ssize_t itemsize) {
         dict args;
@@ -559,11 +559,18 @@ public:
         args["formats"] = std::move(formats);
         args["offsets"] = std::move(offsets);
         args["itemsize"] = pybind11::int_(itemsize);
-        m_ptr = from_args(std::move(args)).release().ptr();
+        m_ptr = from_args(args).release().ptr();
+    }
+
+    explicit dtype(int typenum)
+        : object(detail::npy_api::get().PyArray_DescrFromType_(typenum), stolen_t{}) {
+        if (m_ptr == nullptr) {
+            throw error_already_set();
+        }
     }
 
     /// This is essentially the same as calling numpy.dtype(args) in Python.
-    static dtype from_args(object args) {
+    static dtype from_args(const object &args) {
         PyObject *ptr = nullptr;
         if ((detail::npy_api::get().PyArray_DescrConverter_(args.ptr(), &ptr) == 0) || !ptr) {
             throw error_already_set();
@@ -596,6 +603,23 @@ public:
         return detail::array_descriptor_proxy(m_ptr)->type;
     }
 
+    /// type number of dtype.
+    ssize_t num() const {
+        // Note: The signature, `dtype::num` follows the naming of NumPy's public
+        // Python API (i.e., ``dtype.num``), rather than its internal
+        // C API (``PyArray_Descr::type_num``).
+        return detail::array_descriptor_proxy(m_ptr)->type_num;
+    }
+
+    /// Single character for byteorder
+    char byteorder() const { return detail::array_descriptor_proxy(m_ptr)->byteorder; }
+
+    /// Alignment of the data type
+    int alignment() const { return detail::array_descriptor_proxy(m_ptr)->alignment; }
+
+    /// Flags for the array descriptor
+    char flags() const { return detail::array_descriptor_proxy(m_ptr)->flags; }
+
 private:
     static object _dtype_from_pep3118() {
         static PyObject *obj = module_::import("numpy.core._internal")
@@ -614,7 +638,7 @@ private:
         }
 
         struct field_descr {
-            PYBIND11_STR_TYPE name;
+            pybind11::str name;
             object format;
             pybind11::int_ offset;
         };
@@ -629,7 +653,7 @@ private:
                 continue;
             }
             field_descriptors.push_back(
-                {(PYBIND11_STR_TYPE) name, format.strip_padding(format.itemsize()), offset});
+                {(pybind11::str) name, format.strip_padding(format.itemsize()), offset});
         }
 
         std::sort(field_descriptors.begin(),
@@ -640,9 +664,9 @@ private:
 
         list names, formats, offsets;
         for (auto &descr : field_descriptors) {
-            names.append(descr.name);
-            formats.append(descr.format);
-            offsets.append(descr.offset);
+            names.append(std::move(descr.name));
+            formats.append(std::move(descr.format));
+            offsets.append(std::move(descr.offset));
         }
         return dtype(std::move(names), std::move(formats), std::move(offsets), itemsize);
     }
@@ -940,7 +964,7 @@ protected:
 
     void fail_dim_check(ssize_t dim, const std::string &msg) const {
         throw index_error(msg + ": " + std::to_string(dim) + " (ndim = " + std::to_string(ndim())
-                          + ")");
+                          + ')');
     }
 
     template <typename... Ix>
@@ -1144,11 +1168,11 @@ struct format_descriptor<T, detail::enable_if_t<detail::is_pod_struct<T>::value>
 
 template <size_t N>
 struct format_descriptor<char[N]> {
-    static std::string format() { return std::to_string(N) + "s"; }
+    static std::string format() { return std::to_string(N) + 's'; }
 };
 template <size_t N>
 struct format_descriptor<std::array<char, N>> {
-    static std::string format() { return std::to_string(N) + "s"; }
+    static std::string format() { return std::to_string(N) + 's'; }
 };
 
 template <typename T>
@@ -1288,7 +1312,8 @@ public:
     static pybind11::dtype dtype() {
         list shape;
         array_info<T>::append_extents(shape);
-        return pybind11::dtype::from_args(pybind11::make_tuple(base_descr::dtype(), shape));
+        return pybind11::dtype::from_args(
+            pybind11::make_tuple(base_descr::dtype(), std::move(shape)));
     }
 };
 
@@ -1334,7 +1359,7 @@ PYBIND11_NOINLINE void register_structured_dtype(any_container<field_descriptor>
             pybind11_fail(std::string("NumPy: unsupported field dtype: `") + field.name + "` @ "
                           + tinfo.name());
         }
-        names.append(PYBIND11_STR_TYPE(field.name));
+        names.append(pybind11::str(field.name));
         formats.append(field.descr);
         offsets.append(pybind11::int_(field.offset));
     }
@@ -1526,7 +1551,7 @@ public:
     void *data() const { return p_ptr; }
 
 private:
-    char *p_ptr{0};
+    char *p_ptr{nullptr};
     container_type m_strides;
 };
 
diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 4f6b2cdf4..ce05b6e4e 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -561,14 +561,14 @@ protected:
         for (auto *it = chain_start; it != nullptr; it = it->next) {
             if (options::show_function_signatures()) {
                 if (index > 0) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
                 if (chain) {
                     signatures += std::to_string(++index) + ". ";
                 }
                 signatures += rec->name;
                 signatures += it->signature;
-                signatures += "\n";
+                signatures += '\n';
             }
             if (it->doc && it->doc[0] != '\0' && options::show_user_defined_docstrings()) {
                 // If we're appending another docstring, and aren't printing function signatures,
@@ -577,15 +577,15 @@ protected:
                     if (first_user_def) {
                         first_user_def = false;
                     } else {
-                        signatures += "\n";
+                        signatures += '\n';
                     }
                 }
                 if (options::show_function_signatures()) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
                 signatures += it->doc;
                 if (options::show_function_signatures()) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
             }
         }
@@ -1055,7 +1055,7 @@ protected:
                     msg += it2->signature;
                 }
 
-                msg += "\n";
+                msg += '\n';
             }
             msg += "\nInvoked with: ";
             auto args_ = reinterpret_borrow<tuple>(args_in);
@@ -1817,7 +1817,8 @@ private:
         if (holder_ptr) {
             init_holder_from_existing(v_h, holder_ptr, std::is_copy_constructible<holder_type>());
             v_h.set_holder_constructed();
-        } else if (inst->owned || detail::always_construct_holder<holder_type>::value) {
+        } else if (PYBIND11_SILENCE_MSVC_C4127(detail::always_construct_holder<holder_type>::value)
+                   || inst->owned) {
             new (std::addressof(v_h.holder<holder_type>())) holder_type(v_h.value_ptr<type>());
             v_h.set_holder_constructed();
         }
@@ -2402,7 +2403,8 @@ template <return_value_policy Policy = return_value_policy::reference_internal,
           typename Type,
           typename... Extra>
 iterator make_iterator(Type &value, Extra &&...extra) {
-    return make_iterator<Policy>(std::begin(value), std::end(value), extra...);
+    return make_iterator<Policy>(
+        std::begin(value), std::end(value), std::forward<Extra>(extra)...);
 }
 
 /// Makes an iterator over the keys (`.first`) of a stl map-like container supporting
@@ -2411,7 +2413,8 @@ template <return_value_policy Policy = return_value_policy::reference_internal,
           typename Type,
           typename... Extra>
 iterator make_key_iterator(Type &value, Extra &&...extra) {
-    return make_key_iterator<Policy>(std::begin(value), std::end(value), extra...);
+    return make_key_iterator<Policy>(
+        std::begin(value), std::end(value), std::forward<Extra>(extra)...);
 }
 
 /// Makes an iterator over the values (`.second`) of a stl map-like container supporting
@@ -2420,7 +2423,8 @@ template <return_value_policy Policy = return_value_policy::reference_internal,
           typename Type,
           typename... Extra>
 iterator make_value_iterator(Type &value, Extra &&...extra) {
-    return make_value_iterator<Policy>(std::begin(value), std::end(value), extra...);
+    return make_value_iterator<Policy>(
+        std::begin(value), std::end(value), std::forward<Extra>(extra)...);
 }
 
 template <typename InputType, typename OutputType>
@@ -2485,7 +2489,7 @@ public:
     exception(handle scope, const char *name, handle base = PyExc_Exception) {
         std::string full_name
             = scope.attr("__name__").cast<std::string>() + std::string(".") + name;
-        m_ptr = PyErr_NewException(const_cast<char *>(full_name.c_str()), base.ptr(), NULL);
+        m_ptr = PyErr_NewException(const_cast<char *>(full_name.c_str()), base.ptr(), nullptr);
         if (hasattr(scope, "__dict__") && scope.attr("__dict__").contains(name)) {
             pybind11_fail("Error during initialization: multiple incompatible "
                           "definitions with name \""
@@ -2694,9 +2698,9 @@ get_type_override(const void *this_ptr, const type_info *this_type, const char *
                        d.ptr());
     if (result == nullptr)
         throw error_already_set();
+    Py_DECREF(result);
     if (d["self"].is_none())
         return function();
-    Py_DECREF(result);
 #endif
 
     return override;
diff --git a/include/pybind11/pytypes.h b/include/pybind11/pytypes.h
index f6f5acd18..9584443d1 100644
--- a/include/pybind11/pytypes.h
+++ b/include/pybind11/pytypes.h
@@ -270,10 +270,7 @@ public:
     /// Copy constructor; always increases the reference count
     object(const object &o) : handle(o) { inc_ref(); }
     /// Move constructor; steals the object from ``other`` and preserves its reference count
-    object(object &&other) noexcept {
-        m_ptr = other.m_ptr;
-        other.m_ptr = nullptr;
-    }
+    object(object &&other) noexcept : handle(other) { other.m_ptr = nullptr; }
     /// Destructor; automatically calls `handle::dec_ref()`
     ~object() { dec_ref(); }
 
@@ -636,13 +633,13 @@ inline handle get_function(handle value) {
 inline PyObject *dict_getitemstring(PyObject *v, const char *key) {
     PyObject *kv = nullptr, *rv = nullptr;
     kv = PyUnicode_FromString(key);
-    if (kv == NULL) {
+    if (kv == nullptr) {
         throw error_already_set();
     }
 
     rv = PyDict_GetItemWithError(v, kv);
     Py_DECREF(kv);
-    if (rv == NULL && PyErr_Occurred()) {
+    if (rv == nullptr && PyErr_Occurred()) {
         throw error_already_set();
     }
     return rv;
@@ -650,7 +647,7 @@ inline PyObject *dict_getitemstring(PyObject *v, const char *key) {
 
 inline PyObject *dict_getitem(PyObject *v, PyObject *key) {
     PyObject *rv = PyDict_GetItemWithError(v, key);
-    if (rv == NULL && PyErr_Occurred()) {
+    if (rv == nullptr && PyErr_Occurred()) {
         throw error_already_set();
     }
     return rv;
@@ -1071,12 +1068,12 @@ public:
     Name(const object &o)                                                                         \
         : Parent(check_(o) ? o.inc_ref().ptr() : ConvertFun(o.ptr()), stolen_t{}) {               \
         if (!m_ptr)                                                                               \
-            throw error_already_set();                                                            \
+            throw ::pybind11::error_already_set();                                                \
     }                                                                                             \
     /* NOLINTNEXTLINE(google-explicit-constructor) */                                             \
     Name(object &&o) : Parent(check_(o) ? o.release().ptr() : ConvertFun(o.ptr()), stolen_t{}) {  \
         if (!m_ptr)                                                                               \
-            throw error_already_set();                                                            \
+            throw ::pybind11::error_already_set();                                                \
     }
 
 #define PYBIND11_OBJECT_CVT_DEFAULT(Name, Parent, CheckFun, ConvertFun)                           \
@@ -1276,8 +1273,8 @@ public:
         }
         char *buffer = nullptr;
         ssize_t length = 0;
-        if (PYBIND11_BYTES_AS_STRING_AND_SIZE(temp.ptr(), &buffer, &length)) {
-            pybind11_fail("Unable to extract string contents! (invalid type)");
+        if (PyBytes_AsStringAndSize(temp.ptr(), &buffer, &length) != 0) {
+            throw error_already_set();
         }
         return std::string(buffer, (size_t) length);
     }
@@ -1332,14 +1329,7 @@ public:
     explicit bytes(const pybind11::str &s);
 
     // NOLINTNEXTLINE(google-explicit-constructor)
-    operator std::string() const {
-        char *buffer = nullptr;
-        ssize_t length = 0;
-        if (PYBIND11_BYTES_AS_STRING_AND_SIZE(m_ptr, &buffer, &length)) {
-            pybind11_fail("Unable to extract bytes contents!");
-        }
-        return std::string(buffer, (size_t) length);
-    }
+    operator std::string() const { return string_op<std::string>(); }
 
 #ifdef PYBIND11_HAS_STRING_VIEW
     // enable_if is needed to avoid "ambiguous conversion" errors (see PR #3521).
@@ -1351,15 +1341,18 @@ public:
     // valid so long as the `bytes` instance remains alive and so generally should not outlive the
     // lifetime of the `bytes` instance.
     // NOLINTNEXTLINE(google-explicit-constructor)
-    operator std::string_view() const {
+    operator std::string_view() const { return string_op<std::string_view>(); }
+#endif
+private:
+    template <typename T>
+    T string_op() const {
         char *buffer = nullptr;
         ssize_t length = 0;
-        if (PYBIND11_BYTES_AS_STRING_AND_SIZE(m_ptr, &buffer, &length)) {
-            pybind11_fail("Unable to extract bytes contents!");
+        if (PyBytes_AsStringAndSize(m_ptr, &buffer, &length) != 0) {
+            throw error_already_set();
         }
         return {buffer, static_cast<size_t>(length)};
     }
-#endif
 };
 // Note: breathe >= 4.17.0 will fail to build docs if the below two constructors
 // are included in the doxygen group; close here and reopen after as a workaround
@@ -1370,13 +1363,13 @@ inline bytes::bytes(const pybind11::str &s) {
     if (PyUnicode_Check(s.ptr())) {
         temp = reinterpret_steal<object>(PyUnicode_AsUTF8String(s.ptr()));
         if (!temp) {
-            pybind11_fail("Unable to extract string contents! (encoding issue)");
+            throw error_already_set();
         }
     }
     char *buffer = nullptr;
     ssize_t length = 0;
-    if (PYBIND11_BYTES_AS_STRING_AND_SIZE(temp.ptr(), &buffer, &length)) {
-        pybind11_fail("Unable to extract string contents! (invalid type)");
+    if (PyBytes_AsStringAndSize(temp.ptr(), &buffer, &length) != 0) {
+        throw error_already_set();
     }
     auto obj = reinterpret_steal<object>(PYBIND11_BYTES_FROM_STRING_AND_SIZE(buffer, length));
     if (!obj) {
@@ -1388,8 +1381,8 @@ inline bytes::bytes(const pybind11::str &s) {
 inline str::str(const bytes &b) {
     char *buffer = nullptr;
     ssize_t length = 0;
-    if (PYBIND11_BYTES_AS_STRING_AND_SIZE(b.ptr(), &buffer, &length)) {
-        pybind11_fail("Unable to extract bytes contents!");
+    if (PyBytes_AsStringAndSize(b.ptr(), &buffer, &length) != 0) {
+        throw error_already_set();
     }
     auto obj = reinterpret_steal<object>(PyUnicode_FromStringAndSize(buffer, length));
     if (!obj) {
@@ -1556,8 +1549,8 @@ private:
 class slice : public object {
 public:
     PYBIND11_OBJECT_DEFAULT(slice, object, PySlice_Check)
-    slice(handle start, handle stop, handle step) {
-        m_ptr = PySlice_New(start.ptr(), stop.ptr(), step.ptr());
+    slice(handle start, handle stop, handle step)
+        : object(PySlice_New(start.ptr(), stop.ptr(), step.ptr()), stolen_t{}) {
         if (!m_ptr) {
             pybind11_fail("Could not allocate slice object!");
         }
@@ -1607,7 +1600,7 @@ public:
                      void (*destructor)(PyObject *) = nullptr)
         : object(PyCapsule_New(const_cast<void *>(value), name, destructor), stolen_t{}) {
         if (!m_ptr) {
-            pybind11_fail("Could not allocate capsule object!");
+            throw error_already_set();
         }
     }
 
@@ -1615,34 +1608,44 @@ public:
     capsule(const void *value, void (*destruct)(PyObject *))
         : object(PyCapsule_New(const_cast<void *>(value), nullptr, destruct), stolen_t{}) {
         if (!m_ptr) {
-            pybind11_fail("Could not allocate capsule object!");
+            throw error_already_set();
         }
     }
 
     capsule(const void *value, void (*destructor)(void *)) {
         m_ptr = PyCapsule_New(const_cast<void *>(value), nullptr, [](PyObject *o) {
             auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
-            void *ptr = PyCapsule_GetPointer(o, nullptr);
+            if (destructor == nullptr) {
+                if (PyErr_Occurred()) {
+                    throw error_already_set();
+                }
+                pybind11_fail("Unable to get capsule context");
+            }
+            const char *name = get_name_in_error_scope(o);
+            void *ptr = PyCapsule_GetPointer(o, name);
+            if (ptr == nullptr) {
+                throw error_already_set();
+            }
             destructor(ptr);
         });
 
-        if (!m_ptr) {
-            pybind11_fail("Could not allocate capsule object!");
-        }
-
-        if (PyCapsule_SetContext(m_ptr, (void *) destructor) != 0) {
-            pybind11_fail("Could not set capsule context!");
+        if (!m_ptr || PyCapsule_SetContext(m_ptr, (void *) destructor) != 0) {
+            throw error_already_set();
         }
     }
 
     explicit capsule(void (*destructor)()) {
         m_ptr = PyCapsule_New(reinterpret_cast<void *>(destructor), nullptr, [](PyObject *o) {
-            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, nullptr));
+            const char *name = get_name_in_error_scope(o);
+            auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, name));
+            if (destructor == nullptr) {
+                throw error_already_set();
+            }
             destructor();
         });
 
         if (!m_ptr) {
-            pybind11_fail("Could not allocate capsule object!");
+            throw error_already_set();
         }
     }
 
@@ -1657,8 +1660,7 @@ public:
         const auto *name = this->name();
         T *result = static_cast<T *>(PyCapsule_GetPointer(m_ptr, name));
         if (!result) {
-            PyErr_Clear();
-            pybind11_fail("Unable to extract capsule contents!");
+            throw error_already_set();
         }
         return result;
     }
@@ -1666,12 +1668,37 @@ public:
     /// Replaces a capsule's pointer *without* calling the destructor on the existing one.
     void set_pointer(const void *value) {
         if (PyCapsule_SetPointer(m_ptr, const_cast<void *>(value)) != 0) {
-            PyErr_Clear();
-            pybind11_fail("Could not set capsule pointer");
+            throw error_already_set();
         }
     }
 
-    const char *name() const { return PyCapsule_GetName(m_ptr); }
+    const char *name() const {
+        const char *name = PyCapsule_GetName(m_ptr);
+        if ((name == nullptr) && PyErr_Occurred()) {
+            throw error_already_set();
+        }
+        return name;
+    }
+
+    /// Replaces a capsule's name *without* calling the destructor on the existing one.
+    void set_name(const char *new_name) {
+        if (PyCapsule_SetName(m_ptr, new_name) != 0) {
+            throw error_already_set();
+        }
+    }
+
+private:
+    static const char *get_name_in_error_scope(PyObject *o) {
+        error_scope error_guard;
+
+        const char *name = PyCapsule_GetName(o);
+        if ((name == nullptr) && PyErr_Occurred()) {
+            // write out and consume error raised by call to PyCapsule_GetName
+            PyErr_WriteUnraisable(o);
+        }
+
+        return name;
+    }
 };
 
 class tuple : public object {
@@ -1920,8 +1947,8 @@ public:
         return memoryview::from_buffer(reinterpret_cast<void *>(ptr),
                                        sizeof(T),
                                        format_descriptor<T>::value,
-                                       shape,
-                                       strides,
+                                       std::move(shape),
+                                       std::move(strides),
                                        readonly);
     }
 
@@ -1929,7 +1956,8 @@ public:
     static memoryview from_buffer(const T *ptr,
                                   detail::any_container<ssize_t> shape,
                                   detail::any_container<ssize_t> strides) {
-        return memoryview::from_buffer(const_cast<T *>(ptr), shape, strides, true);
+        return memoryview::from_buffer(
+            const_cast<T *>(ptr), std::move(shape), std::move(strides), true);
     }
 
     /** \rst
diff --git a/include/pybind11/stl.h b/include/pybind11/stl.h
index b66129688..51b57a92b 100644
--- a/include/pybind11/stl.h
+++ b/include/pybind11/stl.h
@@ -79,7 +79,7 @@ struct set_caster {
         for (auto &&value : src) {
             auto value_ = reinterpret_steal<object>(
                 key_conv::cast(forward_like<T>(value), policy, parent));
-            if (!value_ || !s.add(value_)) {
+            if (!value_ || !s.add(std::move(value_))) {
                 return handle();
             }
         }
@@ -372,7 +372,7 @@ struct variant_caster<V<Ts...>> {
     bool load_alternative(handle src, bool convert, type_list<U, Us...>) {
         auto caster = make_caster<U>();
         if (caster.load(src, convert)) {
-            value = cast_op<U>(caster);
+            value = cast_op<U>(std::move(caster));
             return true;
         }
         return load_alternative(src, convert, type_list<Us...>{});
@@ -406,6 +406,9 @@ struct variant_caster<V<Ts...>> {
 #if defined(PYBIND11_HAS_VARIANT)
 template <typename... Ts>
 struct type_caster<std::variant<Ts...>> : variant_caster<std::variant<Ts...>> {};
+
+template <>
+struct type_caster<std::monostate> : public void_caster<std::monostate> {};
 #endif
 
 PYBIND11_NAMESPACE_END(detail)
diff --git a/include/pybind11/stl/filesystem.h b/include/pybind11/stl/filesystem.h
index 93727424c..e26f42177 100644
--- a/include/pybind11/stl/filesystem.h
+++ b/include/pybind11/stl/filesystem.h
@@ -13,21 +13,28 @@
 #include <string>
 
 #ifdef __has_include
-#    if defined(PYBIND11_CPP17) && __has_include(<filesystem>)
-#        include <filesystem>
-#        define PYBIND11_HAS_FILESYSTEM 1
+#    if defined(PYBIND11_CPP17)
+#        if __has_include(<filesystem>) && \
+          PY_VERSION_HEX >= 0x03060000
+#            include <filesystem>
+#            define PYBIND11_HAS_FILESYSTEM 1
+#        elif __has_include(<experimental/filesystem>)
+#            include <experimental/filesystem>
+#            define PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM 1
+#        endif
 #    endif
 #endif
 
-#if !defined(PYBIND11_HAS_FILESYSTEM) && !defined(PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL)
+#if !defined(PYBIND11_HAS_FILESYSTEM) && !defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)           \
+    && !defined(PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL)
 #    error                                                                                        \
-        "#include <filesystem> is not available. (Use -DPYBIND11_HAS_FILESYSTEM_IS_OPTIONAL to ignore.)"
+        "Neither #include <filesystem> nor #include <experimental/filesystem is available. (Use -DPYBIND11_HAS_FILESYSTEM_IS_OPTIONAL to ignore.)"
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
-#if defined(PYBIND11_HAS_FILESYSTEM)
+#if defined(PYBIND11_HAS_FILESYSTEM) || defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)
 template <typename T>
 struct path_caster {
 
@@ -94,9 +101,16 @@ public:
     PYBIND11_TYPE_CASTER(T, const_name("os.PathLike"));
 };
 
+#endif // PYBIND11_HAS_FILESYSTEM || defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)
+
+#if defined(PYBIND11_HAS_FILESYSTEM)
 template <>
 struct type_caster<std::filesystem::path> : public path_caster<std::filesystem::path> {};
-#endif // PYBIND11_HAS_FILESYSTEM
+#elif defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)
+template <>
+struct type_caster<std::experimental::filesystem::path>
+    : public path_caster<std::experimental::filesystem::path> {};
+#endif
 
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
diff --git a/include/pybind11/stl_bind.h b/include/pybind11/stl_bind.h
index 3a3e54dd4..22a29b476 100644
--- a/include/pybind11/stl_bind.h
+++ b/include/pybind11/stl_bind.h
@@ -232,7 +232,7 @@ void vector_modifiers(
     /// Slicing protocol
     cl.def(
         "__getitem__",
-        [](const Vector &v, slice slice) -> Vector * {
+        [](const Vector &v, const slice &slice) -> Vector * {
             size_t start = 0, stop = 0, step = 0, slicelength = 0;
 
             if (!slice.compute(v.size(), &start, &stop, &step, &slicelength)) {
@@ -253,7 +253,7 @@ void vector_modifiers(
 
     cl.def(
         "__setitem__",
-        [](Vector &v, slice slice, const Vector &value) {
+        [](Vector &v, const slice &slice, const Vector &value) {
             size_t start = 0, stop = 0, step = 0, slicelength = 0;
             if (!slice.compute(v.size(), &start, &stop, &step, &slicelength)) {
                 throw error_already_set();
@@ -281,7 +281,7 @@ void vector_modifiers(
 
     cl.def(
         "__delitem__",
-        [](Vector &v, slice slice) {
+        [](Vector &v, const slice &slice) {
             size_t start = 0, stop = 0, step = 0, slicelength = 0;
 
             if (!slice.compute(v.size(), &start, &stop, &step, &slicelength)) {
diff --git a/pyproject.toml b/pyproject.toml
index f80cdc1f4..3ba1b4b22 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,8 +24,10 @@ profile = "black"
 [tool.mypy]
 files = ["pybind11"]
 python_version = "3.6"
-warn_unused_configs = true
 strict = true
+show_error_codes = true
+enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
+warn_unreachable = true
 
 [[tool.mypy.overrides]]
 module = ["ghapi.*", "setuptools.*"]
diff --git a/tests/cross_module_gil_utils.cpp b/tests/cross_module_gil_utils.cpp
index 2e64da052..1436c35d6 100644
--- a/tests/cross_module_gil_utils.cpp
+++ b/tests/cross_module_gil_utils.cpp
@@ -25,8 +25,8 @@ void gil_acquire() { py::gil_scoped_acquire gil; }
 
 constexpr char kModuleName[] = "cross_module_gil_utils";
 
-struct PyModuleDef moduledef
-    = {PyModuleDef_HEAD_INIT, kModuleName, NULL, 0, NULL, NULL, NULL, NULL, NULL};
+struct PyModuleDef moduledef = {
+    PyModuleDef_HEAD_INIT, kModuleName, nullptr, 0, nullptr, nullptr, nullptr, nullptr, nullptr};
 
 } // namespace
 
@@ -34,7 +34,7 @@ extern "C" PYBIND11_EXPORT PyObject *PyInit_cross_module_gil_utils() {
 
     PyObject *m = PyModule_Create(&moduledef);
 
-    if (m != NULL) {
+    if (m != nullptr) {
         static_assert(sizeof(&gil_acquire) == sizeof(void *),
                       "Function pointer must have the same size as void*");
         PyModule_AddObject(
diff --git a/tests/test_numpy_dtypes.cpp b/tests/test_numpy_dtypes.cpp
index dd5b123dc..c25bc042b 100644
--- a/tests/test_numpy_dtypes.cpp
+++ b/tests/test_numpy_dtypes.cpp
@@ -289,8 +289,9 @@ py::list test_dtype_ctors() {
     dict["itemsize"] = py::int_(20);
     list.append(py::dtype::from_args(dict));
     list.append(py::dtype(names, formats, offsets, 20));
-    list.append(py::dtype(py::buffer_info((void *) 0, sizeof(unsigned int), "I", 1)));
-    list.append(py::dtype(py::buffer_info((void *) 0, 0, "T{i:a:f:b:}", 1)));
+    list.append(py::dtype(py::buffer_info((void *) nullptr, sizeof(unsigned int), "I", 1)));
+    list.append(py::dtype(py::buffer_info((void *) nullptr, 0, "T{i:a:f:b:}", 1)));
+    list.append(py::dtype(py::detail::npy_api::NPY_DOUBLE_));
     return list;
 }
 
@@ -440,6 +441,34 @@ TEST_SUBMODULE(numpy_dtypes, m) {
         }
         return list;
     });
+    m.def("test_dtype_num", [dtype_names]() {
+        py::list list;
+        for (const auto &dt_name : dtype_names) {
+            list.append(py::dtype(dt_name).num());
+        }
+        return list;
+    });
+    m.def("test_dtype_byteorder", [dtype_names]() {
+        py::list list;
+        for (const auto &dt_name : dtype_names) {
+            list.append(py::dtype(dt_name).byteorder());
+        }
+        return list;
+    });
+    m.def("test_dtype_alignment", [dtype_names]() {
+        py::list list;
+        for (const auto &dt_name : dtype_names) {
+            list.append(py::dtype(dt_name).alignment());
+        }
+        return list;
+    });
+    m.def("test_dtype_flags", [dtype_names]() {
+        py::list list;
+        for (const auto &dt_name : dtype_names) {
+            list.append(py::dtype(dt_name).flags());
+        }
+        return list;
+    });
     m.def("test_dtype_methods", []() {
         py::list list;
         auto dt1 = py::dtype::of<int32_t>();
@@ -581,5 +610,5 @@ TEST_SUBMODULE(numpy_dtypes, m) {
           []() { PYBIND11_NUMPY_DTYPE(SimpleStruct, bool_, uint_, float_, ldbl_); });
 
     // test_str_leak
-    m.def("dtype_wrapper", [](py::object d) { return py::dtype::from_args(std::move(d)); });
+    m.def("dtype_wrapper", [](const py::object &d) { return py::dtype::from_args(d); });
 }
diff --git a/tests/test_numpy_dtypes.py b/tests/test_numpy_dtypes.py
index 7df60583f..fcfd587b1 100644
--- a/tests/test_numpy_dtypes.py
+++ b/tests/test_numpy_dtypes.py
@@ -160,6 +160,7 @@ def test_dtype(simple_dtype):
         d1,
         np.dtype("uint32"),
         d2,
+        np.dtype("d"),
     ]
 
     assert m.test_dtype_methods() == [
@@ -175,8 +176,13 @@ def test_dtype(simple_dtype):
         np.zeros(1, m.trailing_padding_dtype())
     )
 
+    expected_chars = "bhilqBHILQefdgFDG?MmO"
     assert m.test_dtype_kind() == list("iiiiiuuuuuffffcccbMmO")
-    assert m.test_dtype_char_() == list("bhilqBHILQefdgFDG?MmO")
+    assert m.test_dtype_char_() == list(expected_chars)
+    assert m.test_dtype_num() == [np.dtype(ch).num for ch in expected_chars]
+    assert m.test_dtype_byteorder() == [np.dtype(ch).byteorder for ch in expected_chars]
+    assert m.test_dtype_alignment() == [np.dtype(ch).alignment for ch in expected_chars]
+    assert m.test_dtype_flags() == [chr(np.dtype(ch).flags) for ch in expected_chars]
 
 
 def test_recarray(simple_dtype, packed_dtype):
diff --git a/tests/test_pytypes.cpp b/tests/test_pytypes.cpp
index 1ed237ea2..d1e9b81a7 100644
--- a/tests/test_pytypes.cpp
+++ b/tests/test_pytypes.cpp
@@ -11,6 +11,34 @@
 
 #include <utility>
 
+namespace external {
+namespace detail {
+bool check(PyObject *o) { return PyFloat_Check(o) != 0; }
+
+PyObject *conv(PyObject *o) {
+    PyObject *ret = nullptr;
+    if (PyLong_Check(o)) {
+        double v = PyLong_AsDouble(o);
+        if (!(v == -1.0 && PyErr_Occurred())) {
+            ret = PyFloat_FromDouble(v);
+        }
+    } else {
+        PyErr_SetString(PyExc_TypeError, "Unexpected type");
+    }
+    return ret;
+}
+
+PyObject *default_constructed() { return PyFloat_FromDouble(0.0); }
+} // namespace detail
+class float_ : public py::object {
+    PYBIND11_OBJECT_CVT(float_, py::object, external::detail::check, external::detail::conv)
+
+    float_() : py::object(external::detail::default_constructed(), stolen_t{}) {}
+
+    double get_value() const { return PyFloat_AsDouble(this->ptr()); }
+};
+} // namespace external
+
 TEST_SUBMODULE(pytypes, m) {
     // test_bool
     m.def("get_bool", [] { return py::bool_(false); });
@@ -131,6 +159,15 @@ TEST_SUBMODULE(pytypes, m) {
         return py::capsule([]() { py::print("destructing capsule"); });
     });
 
+    m.def("return_renamed_capsule_with_destructor", []() {
+        py::print("creating capsule");
+        auto cap = py::capsule([]() { py::print("destructing capsule"); });
+        static const char *capsule_name = "test_name1";
+        py::print("renaming capsule");
+        cap.set_name(capsule_name);
+        return cap;
+    });
+
     m.def("return_capsule_with_destructor_2", []() {
         py::print("creating capsule");
         return py::capsule((void *) 1234, [](void *ptr) {
@@ -138,6 +175,17 @@ TEST_SUBMODULE(pytypes, m) {
         });
     });
 
+    m.def("return_renamed_capsule_with_destructor_2", []() {
+        py::print("creating capsule");
+        auto cap = py::capsule((void *) 1234, [](void *ptr) {
+            py::print("destructing capsule: {}"_s.format((size_t) ptr));
+        });
+        static const char *capsule_name = "test_name2";
+        py::print("renaming capsule");
+        cap.set_name(capsule_name);
+        return cap;
+    });
+
     m.def("return_capsule_with_name_and_destructor", []() {
         auto capsule = py::capsule((void *) 12345, "pointer type description", [](PyObject *ptr) {
             if (ptr) {
@@ -545,4 +593,9 @@ TEST_SUBMODULE(pytypes, m) {
         py::detail::accessor_policies::tuple_item::set(o, (py::size_t) 0, s0);
         return o;
     });
+
+    m.def("square_float_", [](const external::float_ &x) -> double {
+        double v = x.get_value();
+        return v * v;
+    });
 }
diff --git a/tests/test_pytypes.py b/tests/test_pytypes.py
index becd1cc8a..5c715ada6 100644
--- a/tests/test_pytypes.py
+++ b/tests/test_pytypes.py
@@ -195,6 +195,19 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_renamed_capsule_with_destructor()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule
+        renaming capsule
+        destructing capsule
+    """
+    )
+
     with capture:
         a = m.return_capsule_with_destructor_2()
         del a
@@ -207,6 +220,19 @@ def test_capsule(capture):
     """
     )
 
+    with capture:
+        a = m.return_renamed_capsule_with_destructor_2()
+        del a
+        pytest.gc_collect()
+    assert (
+        capture.unordered
+        == """
+        creating capsule
+        renaming capsule
+        destructing capsule: 1234
+    """
+    )
+
     with capture:
         a = m.return_capsule_with_name_and_destructor()
         del a
@@ -634,3 +660,8 @@ def test_implementation_details():
     assert m.tuple_item_set_ssize_t() == ("emely", "edmond")
     assert m.tuple_item_get_size_t(tup) == 93
     assert m.tuple_item_set_size_t() == ("candy", "cat")
+
+
+def test_external_float_():
+    r1 = m.square_float_(2.0)
+    assert r1 == 4.0
diff --git a/tests/test_stl.cpp b/tests/test_stl.cpp
index fd1824beb..b56a91953 100644
--- a/tests/test_stl.cpp
+++ b/tests/test_stl.cpp
@@ -37,9 +37,10 @@ struct type_caster<boost::none_t> : void_caster<boost::none_t> {};
 // Test with `std::variant` in C++17 mode, or with `boost::variant` in C++11/14
 #if defined(PYBIND11_HAS_VARIANT)
 using std::variant;
+#    define PYBIND11_TEST_VARIANT 1
 #elif defined(PYBIND11_TEST_BOOST)
 #    include <boost/variant.hpp>
-#    define PYBIND11_HAS_VARIANT 1
+#    define PYBIND11_TEST_VARIANT 1
 using boost::variant;
 
 namespace pybind11 {
@@ -424,7 +425,7 @@ TEST_SUBMODULE(stl, m) {
     m.def("parent_path", [](const std::filesystem::path &p) { return p.parent_path(); });
 #endif
 
-#ifdef PYBIND11_HAS_VARIANT
+#ifdef PYBIND11_TEST_VARIANT
     static_assert(std::is_same<py::detail::variant_caster_visitor::result_type, py::handle>::value,
                   "visitor::result_type is required by boost::variant in C++11 mode");
 
@@ -435,6 +436,9 @@ TEST_SUBMODULE(stl, m) {
         result_type operator()(const std::string &) { return "std::string"; }
         result_type operator()(double) { return "double"; }
         result_type operator()(std::nullptr_t) { return "std::nullptr_t"; }
+#    if defined(PYBIND11_HAS_VARIANT)
+        result_type operator()(std::monostate) { return "std::monostate"; }
+#    endif
     };
 
     // test_variant
@@ -448,6 +452,18 @@ TEST_SUBMODULE(stl, m) {
         using V = variant<int, std::string>;
         return py::make_tuple(V(5), V("Hello"));
     });
+
+#    if defined(PYBIND11_HAS_VARIANT)
+    // std::monostate tests.
+    m.def("load_monostate_variant",
+          [](const variant<std::monostate, int, std::string> &v) -> const char * {
+              return py::detail::visit_helper<variant>::call(visitor(), v);
+          });
+    m.def("cast_monostate_variant", []() {
+        using V = variant<std::monostate, int, std::string>;
+        return py::make_tuple(V{}, V(5), V("Hello"));
+    });
+#    endif
 #endif
 
     // #528: templated constructor
diff --git a/tests/test_stl.py b/tests/test_stl.py
index a3d686689..3dc55230a 100644
--- a/tests/test_stl.py
+++ b/tests/test_stl.py
@@ -263,6 +263,22 @@ def test_variant(doc):
     )
 
 
+@pytest.mark.skipif(
+    not hasattr(m, "load_monostate_variant"), reason="no std::monostate"
+)
+def test_variant_monostate(doc):
+    assert m.load_monostate_variant(None) == "std::monostate"
+    assert m.load_monostate_variant(1) == "int"
+    assert m.load_monostate_variant("1") == "std::string"
+
+    assert m.cast_monostate_variant() == (None, 5, "Hello")
+
+    assert (
+        doc(m.load_monostate_variant)
+        == "load_monostate_variant(arg0: Union[None, int, str]) -> str"
+    )
+
+
 def test_vec_of_reference_wrapper():
     """#171: Can't return reference wrappers (or STL structures containing them)"""
     assert (