pybind/pybind11

noexcept base class methods are not recognized in C++17 mode

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#2234 opened on May 29, 2020

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Description

Issue description

Base class methods attributed with noexcept are not recognized by pybind11 overload system in C++17 mode. The example below compiles just fine in C++11/C++14 mode. Probably it's related to:

The noexcept-specification is a part of the function type and may appear as part of any function declarator. (since C++17)

https://en.cppreference.com/w/cpp/language/noexcept_spec#Explanation

Reproducible example code

#include "pybind11/pybind11.h"
#include "pybind11/embed.h"

namespace py = pybind11;

struct Base {
    int size() const noexcept { return 0; }
};

struct Derived : Base {};

PYBIND11_EMBEDDED_MODULE(example, m) {
  py::class_<Derived>(m, "Derived")
      .def(py::init<>())
      .def("size", &Derived::size);
}

int main() {
  py::scoped_interpreter guard{};
  py::exec(R"(
from example import Derived
d = Derived()
print(d.size())
)");
}

Output (C++17):

terminate called after throwing an instance of 'pybind11::error_already_set'
  what():  TypeError: size(): incompatible function arguments. The following argument types are supported:
    1. (self: Base) -> int

Invoked with: <example.Derived object at 0x7f42fb179ce0>

At:
  <string>(5): <module>

(tried with g++-8 / clang-9)

Contributor guide

Tech stack
cpppython
Domain
backendapi
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++17pybind11 internalstemplate metaprogramming
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Examine pybind11's overload resolution mechanism, particularly how function types are compared. The issue arises because in C++17, noexcept is part of the function type, but pybind11's type traits may not account for it. Look at files like `include/pybind11/pybind11.h` and `include/pybind11/detail/common.h`. Check if the overload matching uses `std::is same` or custom type traits that ignore noexcept. Consider adding a trait to strip noexcept or adjust the matching logic to handle it. Also verify if any existing tests cover C++17 mode and review issue comments for prior investigation.