[BUG]: enum should not be silently converted to float in method call

[codeinred]

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

v2.11.1

Problem description

Enums (including enum class) will be implicitly converted to ints or floats during method calls. This behavior is surprising and can lead to problems where the wrong function is selected during overload resolution.

There should be a way to disable implicit conversion of enums to other types.

Consider the following case:

PYBIND11_MODULE(cmake_example, m) {
    py::enum_<Color>(m, "Color")
        .value("RED", Color::RED)
        .value("GREEN", Color::GREEN)
        .value("BLUE", Color::BLUE);

    m.def("f", [](double, double) { return "f(double, double)"; });
    m.def("f", [](double, Color) { return "f(double, Color)"; });
}

Calling f(0, Color.RED) from python incorrectly calls the f(double, double) overload. If the Color enum were not implicitly convertible, this would not occur.

Backwards Compatibility Concerns

I understand that there may be concerns about backwards compatibility. If these concerns are significant enough, then there should at least be a way to opt out of implicit conversion for enums

Reproducible example code

Full example code (C++):

#include <pybind11/pybind11.h>

namespace py = pybind11;

enum class Color { RED = 0, GREEN = 1, BLUE = 2 };

int add_int(int i, int j) { return i + j; }
double add_float(double x, double y) { return x + y; }

PYBIND11_MODULE(cmake_example, m) {
    m.def("add_int", &add_int);
    m.def("add_float", &add_float);

    py::enum_<Color>(m, "Color")
        .value("RED", Color::RED)
        .value("GREEN", Color::GREEN)
        .value("BLUE", Color::BLUE);

    /// Example of overload failure - calling f(0, Color) in python
    /// returns "f(double, double)" due to implicit conversion
    m.def("f", [](double, double) { return "f(double, double)"; });
    m.def("f", [](double, Color) { return "f(double, Color)"; });
}

Full example code (Python):

import cmake_example as m

# Probable bug - m.Color should _not_ be implicitly convertible to int
# Both of these should throw an invalid argument exception
print(m.add_int(1, m.Color.BLUE))     # Calls add_int, prints 3
print(m.add_float(0.5, m.Color.BLUE)) # Calls add_float, prints 2.5

# Overload case
print(m.f(0.0, m.Color.BLUE)) # Calls f(double, Color) (CORRECT)
print(m.f(0, m.Color.BLUE))   # Calls f(double, double) (INCORRECT)

Output of python:

3
2.5
f(double, Color)
f(double, double)

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

Not a regression

[codeinred]

Based on this test code it seems like implicit conversion to int is intended behavior.

def test_enum_to_int():
    m.test_enum_to_int(m.Flags.Read)
    m.test_enum_to_int(m.ClassWithUnscopedEnum.EMode.EFirstMode)
    m.test_enum_to_int(m.ScopedCharEnum.Positive)
    m.test_enum_to_int(m.ScopedBoolEnum.TRUE)
    # ...

This is fine, but

• Implicit conversion to float is NOT intended behavior,
• There should be a way to opt out of implicit conversion for an enum altogether (implementing this may be easier than fixing implicit conversion to float)

[EmilDohne]

Hey, I just wanted to check if theres an update on this in regards to disabling the implicit conversion from enum to int since I am running into the same problem where I want two separate constructors:

template <typename T>
constructLayer(std::unordered_map<int, py::array_t<T>>& channel_mapping);

template <typename T>
constructLayer(std::unordered_map<Enum::ChannelID, py::array_t<T>>& channel_mapping);

but it appears to always call the first ctor even when calling it with these args from python

# Passing these to the ctor always calls the int overload
image_dict = {}
image_dict[0] = np.full((height, width), 255, np.uint8)
image_dict[1] = np.full((height, width), 0, np.uint8)
image_dict[2] = np.full((height, width), 0, np.uint8)

image_dict_id = {}
image_dict_id[psapi.enum.ChannelID.red] = np.full((height, width), 255, np.uint8)
image_dict_id[psapi.enum.ChannelID.green] = np.full((height, width), 0, np.uint8)
image_dict_id[psapi.enum.ChannelID.blue] = np.full((height, width), 0, np.uint8)