docs: some added notes from @EricCousineau-TRI

Author: henryiii

docs/advanced/cast/index.rst

@@ -1,3 +1,5 @@
+.. _type-conversions:
+
 Type conversions
 ################
 

docs/advanced/classes.rst

@@ -1245,4 +1245,8 @@ You can get the type object from a C++ class that has already been registered us
 You can directly use ``py::type::of(ob)`` to get the type object from any python
 object, just like ``type(ob)`` in Python.
 
+.. note::
+
+    Other types, like ``py::type::of<int>``, do not work, see :ref:`type-conversions`.
+
 .. versionadded:: 2.6

include/pybind11/pytypes.h

@@ -901,7 +901,8 @@ class type : public object {
     explicit type(object ob): type((PyObject*) Py_TYPE(ob.ptr()), borrowed_t{}) {}
 
     /// Convert C++ type to py::type if previously registered. Does not convert
-    //standard types, like int, float. etc. yet.
+    // standard types, like int, float. etc. yet.
+    // See https://github.com/pybind/pybind11/issues/2486
     template<typename T>
     static type of();
 };

tests/test_class.cpp

@@ -139,6 +139,7 @@ TEST_SUBMODULE(class_, m) {
     // test_type
     m.def("check_type", [](int category) {
         // Currently not supported (via a fail at compile time)
+        // See https://github.com/pybind/pybind11/issues/2486
         // if (category == 2)
         //     return py::type::of<int>();
         if (category == 1)

tests/test_class.py

@@ -35,6 +35,7 @@ def test_type():
     assert 'Invalid' in str(execinfo.value)
 
     # Currently not supported
+    # See https://github.com/pybind/pybind11/issues/2486
     # assert m.check_type(2) == int