[BUG] `py::type` constructor from `object` does not throw when passed a non-type object

[phcerdan]

Issue description

py::type(object) constructor is returning an instance, which is unexpected.

This report was suggested by @YannickJadoul in gitter: https://gitter.im/pybind/Lobby?at=5fc11de4b12c2622f909e92f

Reproducible example code

// py::type::of<int> fails at compilation time.
// But it works for any wrapped type.
template<typename T>
using IsCPPType = typename std::enable_if<std::is_base_of<pybind11::detail::type_caster_generic, pybind11::detail::make_caster<T>>::value>::type;

template<typename T>
using IsNotCPPType = typename std::enable_if<!std::is_base_of<pybind11::detail::type_caster_generic, pybind11::detail::make_caster<T>>::value>::type;

template<typename TT>
void def_TValue(
        pybind11::class_<TT, std::unique_ptr<TT>> & py_class,
        IsCPPType<typename TT::Value> * = nullptr) {
    py_class.def_property_readonly_static("TValue",
            [](pybind11::object /* self */ ) {
            return pybind11::type::of<typename TT::Value>();
            });
}
template<typename TT>
void def_TValue(pybind11::class_<TT, std::unique_ptr<TT>> & py_class,
        IsNotCPPType<typename TT::Value> * = nullptr) {
    py_class.def_property_readonly_static("TValue",
            [](pybind11::object /* self */ ) {
            return pybind11::type( // This is returning an instance,not a type, it should throw. <---
            // return pybind11::type::of( // This is the correct way of returning the type.
                        pybind11::cast(typename TT::Value())
                    );
            });
}

template<typename T>
struct foo {
  using Value = T;
}
using FooInt = foo<int>;
using FooWrapped = foo<other_cpp_class>;
py_class_with_int = py::class_<FooInt , std::unique_ptr<FooInt>>(m, "FooInt");
def_TValue(py_class_with_int);
py_class_with_wrapped_class = py::class_<FooWrapped , std::unique_ptr<FooWrapped>>(m, "FooWrapped");
def_TValue(py_class_with_wrapped_class);

print(FooInt().TValue)
0 # unexpected? returns an instance!!
print(FooFloat().TValue)
0.0 # same

print(FooWrapped().TValue)
`<class  'other_cpp_class'>` # correct

After changing pybind11::type( for pybind11::type::of( the result is the correct <class 'int'>

[YannickJadoul]

Additional explanation: I think we decided in that PR (#2364) that py::type(obj) shouldn't work (since there's py::type::of(obj)), but I also thought we made it throw an exception if obj isn't actually a type (cfr. PyType_Check)? Smells like a bug.

[YannickJadoul]

Minimal reproducer:

#include <pybind11/pybind11.h>

namespace py = pybind11;

PYBIND11_MODULE(example, m) {
	py::print(py::type(py::cast(int{})));
}

EDIT: Even minimal-er example

#include <pybind11/pybind11.h>

namespace py = pybind11;

PYBIND11_MODULE(example, m) {
        py::print(py::type(py::int_()));
}

[YannickJadoul]

Got it:

pybind11/include/pybind11/pytypes.h

Lines 825 to 826 in 028812a

	Name(object &&o) : Parent(std::move(o)) \
	{ if (o && !check_(o)) throw PYBIND11_OBJECT_CHECK_FAILED(Name, o); }

Checking the value of the PyObject* in a moved-from py::object is not a great plan ... :-|