[BUG]: 3.0.0rc1 regression: pybind11/cast.h:70:32: error: invalid ‘static_cast’

[HinTak]

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

What version (or hash if on master) of pybind11 are you using?

2.3.0rc1

Problem description

I am trying out the numpy 2.3.0rc1 arm windows support,
and accidentally enabled building skia-python against pybind11 3.0.0rc1 too - https://github.com/kyamagu/skia-python/actions/runs/15240052449 -

It is failing with pybind11's cast.h:

src/skia/Canvas.cpp:239:19:   required from here
        /opt/python/cp310-cp310/lib/python3.10/site-packages/pybind11/include/pybind11/cast.h:70:32: error: invalid ‘static_cast’ from type ‘const SkCanvas::Lattice::RectType* const’ to type ‘pybind11::detail::type_caster_enum_type<SkCanvas::Lattice::RectType>::Underlying’ {aka ‘unsigned char’}
           70 |             return native_enum(static_cast<Underlying>(src)).release();
              |                                ^~~~~~~~~~

I had a quick look at the upgrade guide and nothing jumps out to me yet.

Reproducible example code

Is this a regression? Put the last known working version here if it is.

2.13.6

[rwgk]

What version (or hash if on master) of pybind11 are you using?
2.3.0rc1

Do you mean 3.0.0rc1?

I am trying out the numpy 2.3.0rc1 arm windows support,
and accidentally enabled building skia-python against pybind11 3.0.0rc1 too - https://github.com/kyamagu/skia-python/actions/runs/15240052449 -

I'm getting a 404 clicking on the link. Could you please double-check the link?

I had a quick look at the upgrade guide and nothing jumps out to me yet.

Did you see/try this already?

• pybind11/docs/upgrade.rst

  Lines 103 to 118 in 1c10d5e

  	* In rare cases, a C++ enum may be bound to Python via a
  	:ref:`custom type caster <custom_type_caster>`. In such cases, a
  	template specialization like this may be required:
  	.. code-block:: cpp
  	#if defined(PYBIND11_HAS_NATIVE_ENUM)
  	namespace pybind11::detail {
  	template <typename FancyEnum>
  	struct type_caster_enum_type_enabled<
  	FancyEnum,
  	enable_if_t<is_fancy_enum<FancyEnum>::value>> : std::false_type {};
  	}
  	#endif
  	This specialization is needed only if the custom type caster is templated.

error: invalid ‘static_cast’ from type ‘const SkCanvas::Lattice::RectType* const’

Hm ... a pointer?

A reproducer would be extremely useful.

[HinTak]

Apologies for so many things - yes, it is pybind11 3.0.0rc1 I accidentally enabled when I enabled numpy 2.3.0rc1 . Numpy 2.3.0rc1, or rather, arm64 windows turned out to be quite a change and I had about a dozen changes to get ci to pass, so I deleted the failed ci logs... anyway, the line of code the log referred is

https://github.com/HinTak/skia-python/blob/1547890ce648f3b8a40abe4120acdfea6277b853/src/skia/Canvas.cpp#L239

Where the enum is defined a few lines up:

https://github.com/HinTak/skia-python/blob/1547890ce648f3b8a40abe4120acdfea6277b853/src/skia/Canvas.cpp#L211

Afaic, it is just doing def_readwrite on a py::enum struct member for which the underlying type is "unsigned char" (a very small enum which takes only a handful of values, 256 is enough). The pointer is just the def_readwrite part to write to it. Hope this make sense?

[HinTak]

Anyway, binding against pybind11 2.13.x is passing now, so this is definitely a regression.

[rwgk]

Sorry I don't have a lot of time to spend on this, could you please

1. Try the type_caster_enum_type_enabled specialization as documented in the upgrade guide? — I strongly believe that should work.
2. Reduce your production code into a unit test (and post as a draft PR here)? — This is to help me understand the situation.

Note that the native_enum feature passed testing in a gigantic code base, including many third-party packages (Google, when I was still working there). Therefore your case is likely to be an unusual corner case.

[HinTak]

Okay, thanks for the time. I see it looks similar to #5555 (comment) - I am busy with the original stuff for which this issue comes as a side line, for a few more days. I'll come back to this perhaps in a weeks' time with trying the type caster in 1, and will try to do 2 too.

[henryiii]

Might this be because it used to convert to the underlying integer type, but now it's expecting a py::native_enum caster to be registered?

[rwgk]

Might this be because it used to convert to the underlying integer type, but now it's expecting a py::native_enum caster to be registered?

... no, I think. The error in the issue description is a compiler error. py::native_enum types are registered at runtime.

I looked around:

• https://skia.googlesource.com/skia.git

include/core/SkCanvas.h:

    struct Lattice {
...
        enum RectType : uint8_t {
            kDefault     = 0, //!< draws SkBitmap into lattice rectangle
            kTransparent,     //!< skips lattice rectangle by making it transparent
            kFixedColor,      //!< draws one of fColors into lattice rectangle
        };
...
    };

That looks totally fine.

But looking at the error message again, I beginning to think what's missing is dealing with pointers to enums correctly. And I'm beginning to be surprised that this didn't surface before. (The cast.h code did pass global testing.)

I think we need to add

    template <typename SrcType>                                                 
    static handle cast(const SrcType *src, return_value_policy, handle parent) {

or similar.

[HinTak]

Looking at the skia-python code again, I am not sure it makes sense for it to be def_readwrite - that field is indeed a pointer to enum (with the underlying uint8_t), but it is meant to be an array of enums where the counts are kept nearby in fXCount / fYCount . So the getter and the setter should manipulate all 3 of those values together. That said, I kind of expect pybind11 to be just dereferencing the first value of the array, like it normally does, instead of failing to compile.