feat: py::type::of<T>() and py::type::of(h) (#2364)

* feat: type<T>()

* refactor: using py::type as class

* refactor: py::object as base

* wip: tigher api

* refactor: fix conversion and limit API further

* docs: some added notes from @EricCousineau-TRI

* refactor: use py::type::of

include/pybind11/cast.h

@@ -2204,6 +2204,18 @@ object object_api<Derived>::call(Args &&...args) const {
 
 PYBIND11_NAMESPACE_END(detail)
 
+
+template<typename T>
+type type::of() {
+   static_assert(
+      std::is_base_of<detail::type_caster_generic, detail::make_caster<T>>::value,
+      "py::type::of<T> only supports the case where T is a registered C++ types."
+    );
+
+    return type((PyObject*) detail::get_type_handle(typeid(T), true).ptr(), borrowed_t());
+}
+
+
 #define PYBIND11_MAKE_OPAQUE(...) \
     namespace pybind11 { namespace detail { \
         template<> class type_caster<__VA_ARGS__> : public type_caster_base<__VA_ARGS__> { }; \

include/pybind11/pytypes.h

@@ -19,6 +19,7 @@ PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 /* A few forward declarations */
 class handle; class object;
 class str; class iterator;
+class type;
 struct arg; struct arg_v;
 
 PYBIND11_NAMESPACE_BEGIN(detail)
@@ -890,6 +891,21 @@ private:
     object value = {};
 };
 
+
+
+class type : public object {
+public:
+    PYBIND11_OBJECT(type, object, PyType_Check)
+
+    static type of(handle h) { return type((PyObject*) Py_TYPE(h.ptr()), borrowed_t{}); }
+
+    /// Convert C++ type to py::type if previously registered. Does not convert
+    // standard types, like int, float. etc. yet.
+    // See https://github.com/pybind/pybind11/issues/2486
+    template<typename T>
+    static type of();
+};
+
 class iterable : public object {
 public:
     PYBIND11_OBJECT_DEFAULT(iterable, object, detail::PyIterable_Check)