Fix py::cast from pytype rvalue to pytype

[MaartenBaert]

Previously, py::cast blindly assumed that the destination type was a C++ type rather than a python type when the source type was an rvalue.

Description

The following piece of code:

py::int_ value = py::float_(1.0).cast<py::int_>();

... results in the following bizarre runtime error:

RuntimeError: Unable to cast Python instance of type <class 'float'> to C++ type 'int_'

Whereas this works fine:

py::float_ temp = 1.0;
py::int_ value = temp.cast<py::int_>();

The problem seems to be that in the first case, the py::float_ is an rvalue, which causes pybind to use a different implementation of py::cast which blindly assumes that the destination type is always a C++ type and attempts to use the type_caster mechanism - unlike the normal py::cast implementation which checks the destination type to determine whether to use type_caster or just call the constructor of the destination type.

This commit does three things:

• Add a static assertion to load_type such that if it gets invoked with a python type rather than a C++ type, it will produce a meaningful compile-time error rather than the bizarre runtime error shown above.
• Modify py::cast(object &&) such that it actually checks whether the destination type is a python object, and if so, invokes the move constructor instead of using type_caster.
• Add a test to make sure that the problem is fixed.

Suggested changelog entry:

Fix cast from pytype rvalue to another pytype.

[rwgk]

Looks good to me.
I was looking for @Skylion007's comment that I saw in the email, but cannot find it here (anymore?). FWIW: I'm fine either way, slightly in favor of the straightforward &&.

[Skylion007]

@rwgk Yeah, I looked more closely at the logic and realized all_of any_of would not have helped.