CodeHealth: Enabling clang-tidy google-explicit-constructor (#3250)

* Adding google-explicit-constructor to .clang-tidy

* clang-tidy explicit attr.h (all automatic)

* clang-tidy explicit cast.h (all automatic)

* clang-tidy detail/init.h (1 NOLINT)

* clang-tidy detail/type_caster_base.h (2 NOLINT)

* clang-tidy pybind11.h (7 NOLINT)

* clang-tidy detail/common.h (3 NOLINT)

* clang-tidy detail/descr.h (2 NOLINT)

* clang-tidy pytypes.h (23 NOLINT, only 1 explicit)

* clang-tidy eigen.h (7 NOLINT, 0 explicit)

* Adding 2 explicit in functional.h

* Adding 4 explicit in iostream.h

* clang-tidy numpy.h (1 NOLINT, 1 explicit)

* clang-tidy embed.h (0 NOLINT, 1 explicit)

* clang-tidy tests/local_bindings.h (0 NOLINT, 4 explicit)

* clang-tidy tests/pybind11_cross_module_tests.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/pybind11_tests.h (0 NOLINT, 2 explicit)

* clang-tidy tests/test_buffers.cpp (0 NOLINT, 2 explicit)

* clang-tidy tests/test_builtin_casters.cpp (0 NOLINT, 4 explicit)

* clang-tidy tests/test_class.cpp (0 NOLINT, 6 explicit)

* clang-tidy tests/test_copy_move.cpp (0 NOLINT, 7 explicit)

* clang-tidy tests/test_embed/external_module.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/test_embed/test_interpreter.cpp (0 NOLINT, 1 explicit)

* clang-tidy tests/object.h (0 NOLINT, 2 explicit)

* clang-tidy batch of fully automatic fixes.

* Workaround for MSVC 19.16.27045.0 C++17 Python 2 C++ syntax error.

.clang-tidy

@@ -5,6 +5,7 @@ Checks: '
 cppcoreguidelines-init-variables,
 cppcoreguidelines-slicing,
 clang-analyzer-optin.cplusplus.VirtualCall,
+google-explicit-constructor,
 llvm-namespace-comment,
 misc-misplaced-const,
 misc-non-copyable-objects,

include/pybind11/attr.h

@@ -18,7 +18,9 @@ PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 /// @{
 
 /// Annotation for methods
-struct is_method { handle class_; is_method(const handle &c) : class_(c) { } };
+struct is_method { handle class_;
+    explicit is_method(const handle &c) : class_(c) {}
+};
 
 /// Annotation for operators
 struct is_operator { };
@@ -27,16 +29,24 @@ struct is_operator { };
 struct is_final { };
 
 /// Annotation for parent scope
-struct scope { handle value; scope(const handle &s) : value(s) { } };
+struct scope { handle value;
+    explicit scope(const handle &s) : value(s) {}
+};
 
 /// Annotation for documentation
-struct doc { const char *value; doc(const char *value) : value(value) { } };
+struct doc { const char *value;
+    explicit doc(const char *value) : value(value) {}
+};
 
 /// Annotation for function names
-struct name { const char *value; name(const char *value) : value(value) { } };
+struct name { const char *value;
+    explicit name(const char *value) : value(value) {}
+};
 
 /// Annotation indicating that a function is an overload associated with a given "sibling"
-struct sibling { handle value; sibling(const handle &value) : value(value.ptr()) { } };
+struct sibling { handle value;
+    explicit sibling(const handle &value) : value(value.ptr()) {}
+};
 
 /// Annotation indicating that a class derives from another given type
 template <typename T> struct base {
@@ -70,7 +80,9 @@ struct metaclass {
 };
 
 /// Annotation that marks a class as local to the module:
-struct module_local { const bool value; constexpr module_local(bool v = true) : value(v) { } };
+struct module_local { const bool value;
+    constexpr explicit module_local(bool v = true) : value(v) {}
+};
 
 /// Annotation to mark enums as an arithmetic type
 struct arithmetic { };

include/pybind11/cast.h

@@ -82,7 +82,7 @@ public:
         return caster_t::cast(&src.get(), policy, parent);
     }
     template <typename T> using cast_op_type = std::reference_wrapper<type>;
-    operator std::reference_wrapper<type>() { return cast_op<type &>(subcaster); }
+    explicit operator std::reference_wrapper<type>() { return cast_op<type &>(subcaster); }
 };
 
 #define PYBIND11_TYPE_CASTER(type, py_name)                                                       \
@@ -279,7 +279,7 @@ public:
     }
 
     template <typename T> using cast_op_type = void*&;
-    operator void *&() { return value; }
+    explicit operator void *&() { return value; }
     static constexpr auto name = _("capsule");
 private:
     void *value = nullptr;
@@ -487,8 +487,10 @@ public:
         return StringCaster::cast(StringType(1, src), policy, parent);
     }
 
-    operator CharT*() { return none ? nullptr : const_cast<CharT *>(static_cast<StringType &>(str_caster).c_str()); }
-    operator CharT&() {
+    explicit operator CharT *() {
+        return none ? nullptr : const_cast<CharT *>(static_cast<StringType &>(str_caster).c_str());
+    }
+    explicit operator CharT &() {
         if (none)
             throw value_error("Cannot convert None to a character");
 
@@ -581,8 +583,8 @@ public:
 
     template <typename T> using cast_op_type = type;
 
-    operator type() & { return implicit_cast(indices{}); }
-    operator type() && { return std::move(*this).implicit_cast(indices{}); }
+    explicit operator type() & { return implicit_cast(indices{}); }
+    explicit operator type() && { return std::move(*this).implicit_cast(indices{}); }
 
 protected:
     template <size_t... Is>

include/pybind11/detail/common.h

@@ -918,6 +918,7 @@ public:
 
     // Implicit conversion constructor from any arbitrary container type with values convertible to T
     template <typename Container, typename = enable_if_t<std::is_convertible<decltype(*std::begin(std::declval<const Container &>())), T>::value>>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     any_container(const Container &c) : any_container(std::begin(c), std::end(c)) { }
 
     // initializer_list's aren't deducible, so don't get matched by the above template; we need this
@@ -926,9 +927,11 @@ public:
     any_container(const std::initializer_list<TIn> &c) : any_container(c.begin(), c.end()) { }
 
     // Avoid copying if given an rvalue vector of the correct type.
+    // NOLINTNEXTLINE(google-explicit-constructor)
     any_container(std::vector<T> &&v) : v(std::move(v)) { }
 
     // Moves the vector out of an rvalue any_container
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator std::vector<T> &&() && { return std::move(v); }
 
     // Dereferencing obtains a reference to the underlying vector

include/pybind11/detail/descr.h

@@ -26,12 +26,14 @@ struct descr {
     char text[N + 1]{'\0'};
 
     constexpr descr() = default;
+    // NOLINTNEXTLINE(google-explicit-constructor)
     constexpr descr(char const (&s)[N+1]) : descr(s, make_index_sequence<N>()) { }
 
     template <size_t... Is>
     constexpr descr(char const (&s)[N+1], index_sequence<Is...>) : text{s[Is]..., '\0'} { }
 
     template <typename... Chars>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     constexpr descr(char c, Chars... cs) : text{c, static_cast<char>(cs)..., '\0'} { }
 
     static constexpr std::array<const std::type_info *, sizeof...(Ts) + 1> types() {

include/pybind11/detail/init.h

@@ -23,7 +23,7 @@ public:
     }
 
     template <typename> using cast_op_type = value_and_holder &;
-    operator value_and_holder &() { return *value; }
+    explicit operator value_and_holder &() { return *value; }
     static constexpr auto name = _<value_and_holder>();
 
 private:
@@ -222,7 +222,8 @@ template <typename Func, typename Return, typename... Args>
 struct factory<Func, void_type (*)(), Return(Args...)> {
     remove_reference_t<Func> class_factory;
 
-    factory(Func &&f) : class_factory(std::forward<Func>(f)) { }
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    factory(Func &&f) : class_factory(std::forward<Func>(f)) {}
 
     // The given class either has no alias or has no separate alias factory;
     // this always constructs the class itself.  If the class is registered with an alias

include/pybind11/detail/type_caster_base.h

@@ -225,7 +225,7 @@ struct value_and_holder {
     value_and_holder() = default;
 
     // Used for past-the-end iterator
-    value_and_holder(size_t index) : index{index} {}
+    explicit value_and_holder(size_t index) : index{index} {}
 
     template <typename V = void> V *&value_ptr() const {
         return reinterpret_cast<V *&>(vh[0]);
@@ -274,7 +274,8 @@ private:
     const type_vec &tinfo;
 
 public:
-    values_and_holders(instance *inst) : inst{inst}, tinfo(all_type_info(Py_TYPE(inst))) {}
+    explicit values_and_holders(instance *inst)
+        : inst{inst}, tinfo(all_type_info(Py_TYPE(inst))) {}
 
     struct iterator {
     private:
@@ -290,7 +291,8 @@ public:
                  0 /* index */)
         {}
         // Past-the-end iterator:
-        iterator(size_t end) : curr(end) {}
+        explicit iterator(size_t end) : curr(end) {}
+
     public:
         bool operator==(const iterator &other) const { return curr.index == other.curr.index; }
         bool operator!=(const iterator &other) const { return curr.index != other.curr.index; }
@@ -491,11 +493,11 @@ inline PyObject *make_new_instance(PyTypeObject *type);
 
 class type_caster_generic {
 public:
-    PYBIND11_NOINLINE type_caster_generic(const std::type_info &type_info)
-        : typeinfo(get_type_info(type_info)), cpptype(&type_info) { }
+    PYBIND11_NOINLINE explicit type_caster_generic(const std::type_info &type_info)
+        : typeinfo(get_type_info(type_info)), cpptype(&type_info) {}
 
-    type_caster_generic(const type_info *typeinfo)
-        : typeinfo(typeinfo), cpptype(typeinfo ? typeinfo->cpptype : nullptr) { }
+    explicit type_caster_generic(const type_info *typeinfo)
+        : typeinfo(typeinfo), cpptype(typeinfo ? typeinfo->cpptype : nullptr) {}
 
     bool load(handle src, bool convert) {
         return load_impl<type_caster_generic>(src, convert);
@@ -923,7 +925,9 @@ public:
 
     template <typename T> using cast_op_type = detail::cast_op_type<T>;
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator itype*() { return (type *) value; }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator itype&() { if (!value) throw reference_cast_error(); return *((itype *) value); }
 
 protected:

include/pybind11/eigen.h

@@ -61,6 +61,7 @@ template <bool EigenRowMajor> struct EigenConformable {
     EigenDStride stride{0, 0};      // Only valid if negativestrides is false!
     bool negativestrides = false;   // If true, do not use stride!
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     EigenConformable(bool fits = false) : conformable{fits} {}
     // Matrix type:
     EigenConformable(EigenIndex r, EigenIndex c,
@@ -88,6 +89,7 @@ template <bool EigenRowMajor> struct EigenConformable {
             (props::outer_stride == Eigen::Dynamic || props::outer_stride == stride.outer() ||
                 (EigenRowMajor ? rows : cols) == 1);
     }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator bool() const { return conformable; }
 };
 
@@ -326,8 +328,11 @@ public:
 
     static constexpr auto name = props::descriptor;
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type*() { return &value; }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type&() { return value; }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type&&() && { return std::move(value); }
     template <typename T> using cast_op_type = movable_cast_op_type<T>;
 
@@ -451,7 +456,9 @@ public:
         return true;
     }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type*() { return ref.get(); }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type&() { return *ref; }
     template <typename _T> using cast_op_type = pybind11::detail::cast_op_type<_T>;
 

include/pybind11/embed.h

@@ -260,10 +260,10 @@ inline void finalize_interpreter() {
  \endrst */
 class scoped_interpreter {
 public:
-    scoped_interpreter(bool init_signal_handlers = true,
-                       int argc = 0,
-                       const char *const *argv = nullptr,
-                       bool add_program_dir_to_path = true) {
+    explicit scoped_interpreter(bool init_signal_handlers = true,
+                                int argc = 0,
+                                const char *const *argv = nullptr,
+                                bool add_program_dir_to_path = true) {
         initialize_interpreter(init_signal_handlers, argc, argv, add_program_dir_to_path);
     }
 

include/pybind11/functional.h

@@ -69,6 +69,10 @@ public:
         // ensure GIL is held during functor destruction
         struct func_handle {
             function f;
+#if !(defined(_MSC_VER) && _MSC_VER == 1916 && defined(PYBIND11_CPP17) && PY_MAJOR_VERSION < 3)
+            // This triggers a syntax error under very special conditions (very weird indeed).
+            explicit
+#endif
             func_handle(function &&f_) noexcept : f(std::move(f_)) {}
             func_handle(const func_handle &f_) { operator=(f_); }
             func_handle &operator=(const func_handle &f_) {
@@ -85,7 +89,7 @@ public:
         // to emulate 'move initialization capture' in C++11
         struct func_wrapper {
             func_handle hfunc;
-            func_wrapper(func_handle &&hf) noexcept : hfunc(std::move(hf)) {}
+            explicit func_wrapper(func_handle &&hf) noexcept : hfunc(std::move(hf)) {}
             Return operator()(Args... args) const {
                 gil_scoped_acquire acq;
                 object retval(hfunc.f(std::forward<Args>(args)...));

include/pybind11/iostream.h

@@ -123,7 +123,7 @@ private:
     }
 
 public:
-    pythonbuf(const object &pyostream, size_t buffer_size = 1024)
+    explicit pythonbuf(const object &pyostream, size_t buffer_size = 1024)
         : buf_size(buffer_size), d_buffer(new char[buf_size]), pywrite(pyostream.attr("write")),
           pyflush(pyostream.attr("flush")) {
         setp(d_buffer.get(), d_buffer.get() + buf_size - 1);
@@ -171,8 +171,9 @@ protected:
     detail::pythonbuf buffer;
 
 public:
-    scoped_ostream_redirect(std::ostream &costream  = std::cout,
-                            const object &pyostream = module_::import("sys").attr("stdout"))
+    explicit scoped_ostream_redirect(std::ostream &costream = std::cout,
+                                     const object &pyostream
+                                     = module_::import("sys").attr("stdout"))
         : costream(costream), buffer(pyostream) {
         old = costream.rdbuf(&buffer);
     }
@@ -201,8 +202,9 @@ public:
 \endrst */
 class scoped_estream_redirect : public scoped_ostream_redirect {
 public:
-    scoped_estream_redirect(std::ostream &costream  = std::cerr,
-                            const object &pyostream = module_::import("sys").attr("stderr"))
+    explicit scoped_estream_redirect(std::ostream &costream = std::cerr,
+                                     const object &pyostream
+                                     = module_::import("sys").attr("stderr"))
         : scoped_ostream_redirect(costream, pyostream) {}
 };
 
@@ -217,7 +219,7 @@ class OstreamRedirect {
     std::unique_ptr<scoped_estream_redirect> redirect_stderr;
 
 public:
-    OstreamRedirect(bool do_stdout = true, bool do_stderr = true)
+    explicit OstreamRedirect(bool do_stdout = true, bool do_stderr = true)
         : do_stdout_(do_stdout), do_stderr_(do_stderr) {}
 
     void enter() {

include/pybind11/numpy.h

@@ -477,7 +477,7 @@ public:
         m_ptr = from_args(pybind11::str(format)).release().ptr();
     }
 
-    dtype(const char *format) : dtype(std::string(format)) { }
+    explicit dtype(const char *format) : dtype(std::string(format)) {}
 
     dtype(list names, list formats, list offsets, ssize_t itemsize) {
         dict args;
@@ -894,6 +894,7 @@ public:
         if (!is_borrowed) Py_XDECREF(h.ptr());
     }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     array_t(const object &o) : array(raw_array_t(o.ptr()), stolen_t{}) {
         if (!m_ptr) throw error_already_set();
     }

include/pybind11/pybind11.h

@@ -81,10 +81,12 @@ PYBIND11_NAMESPACE_END(detail)
 class cpp_function : public function {
 public:
     cpp_function() = default;
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(std::nullptr_t) { }
 
     /// Construct a cpp_function from a vanilla function pointer
     template <typename Return, typename... Args, typename... Extra>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Return (*f)(Args...), const Extra&... extra) {
         initialize(f, f, extra...);
     }
@@ -92,6 +94,7 @@ public:
     /// Construct a cpp_function from a lambda function (possibly with internal state)
     template <typename Func, typename... Extra,
               typename = detail::enable_if_t<detail::is_lambda<Func>::value>>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Func &&f, const Extra&... extra) {
         initialize(std::forward<Func>(f),
                    (detail::function_signature_t<Func> *) nullptr, extra...);
@@ -99,6 +102,7 @@ public:
 
     /// Construct a cpp_function from a class method (non-const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Return (Class::*f)(Arg...), const Extra&... extra) {
         initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(std::forward<Arg>(args)...); },
                    (Return (*) (Class *, Arg...)) nullptr, extra...);
@@ -108,6 +112,7 @@ public:
     /// A copy of the overload for non-const functions without explicit ref-qualifier
     /// but with an added `&`.
     template <typename Return, typename Class, typename... Arg, typename... Extra>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Return (Class::*f)(Arg...)&, const Extra&... extra) {
         initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(args...); },
                    (Return (*) (Class *, Arg...)) nullptr, extra...);
@@ -115,6 +120,7 @@ public:
 
     /// Construct a cpp_function from a class method (const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Return (Class::*f)(Arg...) const, const Extra&... extra) {
         initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(std::forward<Arg>(args)...); },
                    (Return (*)(const Class *, Arg ...)) nullptr, extra...);
@@ -124,6 +130,7 @@ public:
     /// A copy of the overload for const functions without explicit ref-qualifier
     /// but with an added `&`.
     template <typename Return, typename Class, typename... Arg, typename... Extra>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     cpp_function(Return (Class::*f)(Arg...) const&, const Extra&... extra) {
         initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(args...); },
                    (Return (*)(const Class *, Arg ...)) nullptr, extra...);
@@ -2034,7 +2041,7 @@ template <return_value_policy Policy = return_value_policy::reference_internal,
 template <typename InputType, typename OutputType> void implicitly_convertible() {
     struct set_flag {
         bool &flag;
-        set_flag(bool &flag_) : flag(flag_) { flag_ = true; }
+        explicit set_flag(bool &flag_) : flag(flag_) { flag_ = true; }
         ~set_flag() { flag = false; }
     };
     auto implicit_caster = [](PyObject *obj, PyTypeObject *type) -> PyObject * {

include/pybind11/pytypes.h

@@ -178,6 +178,7 @@ public:
     /// The default constructor creates a handle with a ``nullptr``-valued pointer
     handle() = default;
     /// Creates a ``handle`` from the given raw Python object pointer
+    // NOLINTNEXTLINE(google-explicit-constructor)
     handle(PyObject *ptr) : m_ptr(ptr) { } // Allow implicit conversion from PyObject*
 
     /// Return the underlying ``PyObject *`` pointer
@@ -612,6 +613,7 @@ public:
         return obj.contains(key);
     }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator object() const { return get_cache(); }
     PyObject *ptr() const { return get_cache().ptr(); }
     template <typename T> T cast() const { return get_cache().template cast<T>(); }
@@ -761,6 +763,7 @@ template <typename T>
 struct arrow_proxy {
     T value;
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     arrow_proxy(T &&value) noexcept : value(std::move(value)) { }
     T *operator->() const { return &value; }
 };
@@ -909,14 +912,17 @@ PYBIND11_NAMESPACE_END(detail)
         bool check() const { return m_ptr != nullptr && (CheckFun(m_ptr) != 0); } \
         static bool check_(handle h) { return h.ptr() != nullptr && CheckFun(h.ptr()); } \
         template <typename Policy_> \
+        /* NOLINTNEXTLINE(google-explicit-constructor) */ \
         Name(const ::pybind11::detail::accessor<Policy_> &a) : Name(object(a)) { }
 
 #define PYBIND11_OBJECT_CVT(Name, Parent, CheckFun, ConvertFun) \
     PYBIND11_OBJECT_COMMON(Name, Parent, CheckFun) \
     /* This is deliberately not 'explicit' to allow implicit conversion from object: */ \
+    /* NOLINTNEXTLINE(google-explicit-constructor) */ \
     Name(const object &o) \
     : Parent(check_(o) ? o.inc_ref().ptr() : ConvertFun(o.ptr()), stolen_t{}) \
     { if (!m_ptr) throw 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(); }
@@ -933,8 +939,10 @@ PYBIND11_NAMESPACE_END(detail)
 #define PYBIND11_OBJECT(Name, Parent, CheckFun) \
     PYBIND11_OBJECT_COMMON(Name, Parent, CheckFun) \
     /* This is deliberately not 'explicit' to allow implicit conversion from object: */ \
+    /* NOLINTNEXTLINE(google-explicit-constructor) */ \
     Name(const object &o) : Parent(o) \
     { if (m_ptr && !check_(m_ptr)) throw PYBIND11_OBJECT_CHECK_FAILED(Name, m_ptr); } \
+    /* NOLINTNEXTLINE(google-explicit-constructor) */ \
     Name(object &&o) : Parent(std::move(o)) \
     { if (m_ptr && !check_(m_ptr)) throw PYBIND11_OBJECT_CHECK_FAILED(Name, m_ptr); }
 
@@ -1056,11 +1064,13 @@ public:
     }
 
     // 'explicit' is explicitly omitted from the following constructors to allow implicit conversion to py::str from C++ string-like objects
+    // NOLINTNEXTLINE(google-explicit-constructor)
     str(const char *c = "")
         : object(PyUnicode_FromString(c), stolen_t{}) {
         if (!m_ptr) pybind11_fail("Could not allocate string object!");
     }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     str(const std::string &s) : str(s.data(), s.size()) { }
 
     explicit str(const bytes &b);
@@ -1071,6 +1081,7 @@ public:
     \endrst */
     explicit str(handle h) : object(raw_str(h.ptr()), stolen_t{}) { if (!m_ptr) throw error_already_set(); }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator std::string() const {
         object temp = *this;
         if (PyUnicode_Check(m_ptr)) {
@@ -1118,6 +1129,7 @@ public:
     PYBIND11_OBJECT(bytes, object, PYBIND11_BYTES_CHECK)
 
     // Allow implicit conversion:
+    // NOLINTNEXTLINE(google-explicit-constructor)
     bytes(const char *c = "")
         : object(PYBIND11_BYTES_FROM_STRING(c), stolen_t{}) {
         if (!m_ptr) pybind11_fail("Could not allocate bytes object!");
@@ -1130,10 +1142,12 @@ public:
     }
 
     // Allow implicit conversion:
+    // NOLINTNEXTLINE(google-explicit-constructor)
     bytes(const std::string &s) : bytes(s.data(), s.size()) { }
 
     explicit bytes(const pybind11::str &s);
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator std::string() const {
         char *buffer = nullptr;
         ssize_t length = 0;
@@ -1222,7 +1236,9 @@ public:
     PYBIND11_OBJECT_CVT(bool_, object, PyBool_Check, raw_bool)
     bool_() : object(Py_False, borrowed_t{}) { }
     // Allow implicit conversion from and to `bool`:
+    // NOLINTNEXTLINE(google-explicit-constructor)
     bool_(bool value) : object(value ? Py_True : Py_False, borrowed_t{}) { }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator bool() const { return (m_ptr != nullptr) && PyLong_AsLong(m_ptr) != 0; }
 
 private:
@@ -1261,6 +1277,7 @@ public:
     // Allow implicit conversion from C++ integral types:
     template <typename T,
               detail::enable_if_t<std::is_integral<T>::value, int> = 0>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     int_(T value) {
         if (PYBIND11_SILENCE_MSVC_C4127(sizeof(T) <= sizeof(long))) {
             if (std::is_signed<T>::value)
@@ -1278,6 +1295,7 @@ public:
 
     template <typename T,
               detail::enable_if_t<std::is_integral<T>::value, int> = 0>
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator T() const {
         return std::is_unsigned<T>::value
             ? detail::as_unsigned<T>(m_ptr)
@@ -1291,13 +1309,17 @@ class float_ : public object {
 public:
     PYBIND11_OBJECT_CVT(float_, object, PyFloat_Check, PyNumber_Float)
     // Allow implicit conversion from float/double:
+    // NOLINTNEXTLINE(google-explicit-constructor)
     float_(float value) : object(PyFloat_FromDouble((double) value), stolen_t{}) {
         if (!m_ptr) pybind11_fail("Could not allocate float object!");
     }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     float_(double value = .0) : object(PyFloat_FromDouble((double) value), stolen_t{}) {
         if (!m_ptr) pybind11_fail("Could not allocate float object!");
     }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator float() const { return (float) PyFloat_AsDouble(m_ptr); }
+    // NOLINTNEXTLINE(google-explicit-constructor)
     operator double() const { return (double) PyFloat_AsDouble(m_ptr); }
 };
 
@@ -1372,7 +1394,7 @@ public:
             pybind11_fail("Could not set capsule context!");
     }
 
-    capsule(void (*destructor)()) {
+    explicit capsule(void (*destructor)()) {
         m_ptr = PyCapsule_New(reinterpret_cast<void *>(destructor), nullptr, [](PyObject *o) {
             auto destructor = reinterpret_cast<void (*)()>(PyCapsule_GetPointer(o, nullptr));
             destructor();
@@ -1382,6 +1404,7 @@ public:
             pybind11_fail("Could not allocate capsule object!");
     }
 
+    // NOLINTNEXTLINE(google-explicit-constructor)
     template <typename T> operator T *() const {
         return get_pointer<T>();
     }

tests/local_bindings.h

@@ -6,7 +6,7 @@
 /// Simple class used to test py::local:
 template <int> class LocalBase {
 public:
-    LocalBase(int i) : i(i) { }
+    explicit LocalBase(int i) : i(i) { }
     int i = -1;
 };
 
@@ -75,11 +75,11 @@ py::class_<T> bind_local(Args && ...args) {
 namespace pets {
 class Pet {
 public:
-    Pet(std::string name) : name_(std::move(name)) {}
+    explicit Pet(std::string name) : name_(std::move(name)) {}
     std::string name_;
     const std::string &name() const { return name_; }
 };
 } // namespace pets
 
-struct MixGL { int i; MixGL(int i) : i{i} {} };
-struct MixGL2 { int i; MixGL2(int i) : i{i} {} };
+struct MixGL { int i; explicit MixGL(int i) : i{i} {} };
+struct MixGL2 { int i; explicit MixGL2(int i) : i{i} {} };

tests/object.h

@@ -65,7 +65,7 @@ public:
     ref() : m_ptr(nullptr) { print_default_created(this); track_default_created((ref_tag*) this); }
 
     /// Construct a reference from a pointer
-    ref(T *ptr) : m_ptr(ptr) {
+    explicit ref(T *ptr) : m_ptr(ptr) {
         if (m_ptr) ((Object *) m_ptr)->incRef();
 
         print_created(this, "from pointer", m_ptr); track_created((ref_tag*) this, "from pointer");
@@ -165,7 +165,7 @@ public:
     const T& operator*() const { return *m_ptr; }
 
     /// Return a pointer to the referenced object
-    operator T* () { return m_ptr; }
+    explicit operator T* () { return m_ptr; }
 
     /// Return a const pointer to the referenced object
     T* get_ptr() { return m_ptr; }

tests/pybind11_cross_module_tests.cpp

@@ -123,7 +123,7 @@ PYBIND11_MODULE(pybind11_cross_module_tests, m) {
 
     class Dog : public pets::Pet {
     public:
-        Dog(std::string name) : Pet(std::move(name)) {}
+        explicit Dog(std::string name) : Pet(std::move(name)) {}
     };
     py::class_<pets::Pet>(m, "Pet", py::module_local())
         .def("name", &pets::Pet::name);

tests/pybind11_tests.h

@@ -16,7 +16,7 @@ class test_initializer {
     using Initializer = void (*)(py::module_ &);
 
 public:
-    test_initializer(Initializer init);
+    explicit test_initializer(Initializer init);
     test_initializer(const char *submodule_name, Initializer init);
 };
 
@@ -32,7 +32,7 @@ struct UnregisteredType { };
 class UserType {
 public:
     UserType() = default;
-    UserType(int i) : i(i) { }
+    explicit UserType(int i) : i(i) { }
 
     int value() const { return i; }
     void set(int set) { i = set; }

tests/test_buffers.cpp

@@ -122,7 +122,7 @@ TEST_SUBMODULE(buffers, m) {
     // test_inherited_protocol
     class SquareMatrix : public Matrix {
     public:
-        SquareMatrix(py::ssize_t n) : Matrix(n, n) { }
+        explicit SquareMatrix(py::ssize_t n) : Matrix(n, n) {}
     };
     // Derived classes inherit the buffer protocol and the buffer access function
     py::class_<SquareMatrix, Matrix>(m, "SquareMatrix")
@@ -173,7 +173,7 @@ TEST_SUBMODULE(buffers, m) {
 
     struct BufferReadOnly {
         const uint8_t value = 0;
-        BufferReadOnly(uint8_t value): value(value) {}
+        explicit BufferReadOnly(uint8_t value) : value(value) {}
 
         py::buffer_info get_buffer_info() {
             return py::buffer_info(&value, 1);

tests/test_builtin_casters.cpp

@@ -25,16 +25,28 @@ class type_caster<ConstRefCasted> {
   // cast operator.
   bool load(handle, bool) { return true; }
 
-  operator ConstRefCasted &&() {
+  explicit operator ConstRefCasted &&() {
       value = {1};
       // NOLINTNEXTLINE(performance-move-const-arg)
       return std::move(value);
   }
-  operator ConstRefCasted&() { value = {2}; return value; }
-  operator ConstRefCasted*() { value = {3}; return &value; }
+  explicit operator ConstRefCasted &() {
+      value = {2};
+      return value;
+  }
+  explicit operator ConstRefCasted *() {
+      value = {3};
+      return &value;
+  }
 
-  operator const ConstRefCasted&() { value = {4}; return value; }
-  operator const ConstRefCasted*() { value = {5}; return &value; }
+  explicit operator const ConstRefCasted &() {
+      value = {4};
+      return value;
+  }
+  explicit operator const ConstRefCasted *() {
+      value = {5};
+      return &value;
+  }
 
   // custom cast_op to explicitly propagate types to the conversion operators.
   template <typename T_>