[BUG]: Conditionally enabled member functions using concepts leads to compiler error

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

2.10.4

Problem description

See code example below, which leads to the following compiler error:

foo.cpp: In function 'void pybind11_init_bug(pybind11::module_&)':
foo.cpp:26:13: error: no matching function for call to 'pybind11::class_<Foo<int> >::def(const char [4], <unresolved overloaded function type>)'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from foo.cpp:2:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1567:13: note: candidate: 'template<class Func, class ... Extra> pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const char*, Func&&, const Extra& ...) [with Func = Func; Extra = {Extra ...}; type_ = Foo<int>; options = {}]'
 1567 |     class_ &def(const char *name_, Func &&f, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1567:13: note:   template argument deduction/substitution failed:
foo.cpp:26:13: note:   couldn't deduce template parameter 'Func'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from foo.cpp:2:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1592:13: note: candidate: 'template<class T, class ... Extra, typename std::enable_if<T::op_enable_if_hook, int>::type <anonymous> > pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const T&, const Extra& ...) [with T = T; Extra = {Extra ...}; typename std::enable_if<T::op_enable_if_hook, int>::type <anonymous> = <anonymous>; type_ = Foo<int>; options = {}]'
 1592 |     class_ &def(const T &op, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1592:13: note:   template argument deduction/substitution failed:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1591:95: error: 'op_enable_if_hook' is not a member of 'char [4]'
 1591 |     template <typename T, typename... Extra, detail::enable_if_t<T::op_enable_if_hook, int> = 0>
      |                                                                                               ^
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1604:13: note: candidate: 'template<class ... Args, class ... Extra> pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const pybind11::detail::initimpl::constructor<Args ...>&, const Extra& ...) [with Args = {Args ...}; Extra = {Extra ...}; type_ = Foo<int>; options = {}]'
 1604 |     class_ &def(const detail::initimpl::constructor<Args...> &init, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1604:13: note:   template argument deduction/substitution failed:
foo.cpp:26:13: note:   mismatched types 'const pybind11::detail::initimpl::constructor<Args ...>' and 'const char [4]'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from foo.cpp:2:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1611:13: note: candidate: 'template<class ... Args, class ... Extra> pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const pybind11::detail::initimpl::alias_constructor<Args ...>&, const Extra& ...) [with Args = {Args ...}; Extra = {Extra ...}; type_ = Foo<int>; options = {}]'
 1611 |     class_ &def(const detail::initimpl::alias_constructor<Args...> &init, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1611:13: note:   template argument deduction/substitution failed:
foo.cpp:26:13: note:   mismatched types 'const pybind11::detail::initimpl::alias_constructor<Args ...>' and 'const char [4]'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from foo.cpp:2:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1618:13: note: candidate: 'template<class ... Args, class ... Extra> pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(pybind11::detail::initimpl::factory<Args ...>&&, const Extra& ...) [with Args = {Args ...}; Extra = {Extra ...}; type_ = Foo<int>; options = {}]'
 1618 |     class_ &def(detail::initimpl::factory<Args...> &&init, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1618:13: note:   template argument deduction/substitution failed:
foo.cpp:26:13: note:   mismatched types 'pybind11::detail::initimpl::factory<Args ...>' and 'const char [4]'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
In file included from foo.cpp:2:
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1624:13: note: candidate: 'template<class ... Args, class ... Extra> pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(pybind11::detail::initimpl::pickle_factory<Args ...>&&, const Extra& ...) [with Args = {Args ...}; Extra = {Extra ...}; type_ = Foo<int>; options = {}]'
 1624 |     class_ &def(detail::initimpl::pickle_factory<Args...> &&pf, const Extra &...extra) {
      |             ^~~
C:/xxx/.conda/envs/p311/Lib/site-packages/pybind11/include/pybind11/pybind11.h:1624:13: note:   template argument deduction/substitution failed:
foo.cpp:26:13: note:   mismatched types 'pybind11::detail::initimpl::pickle_factory<Args ...>' and 'const char [4]'
   25 |     py::class_<Foo<int>>(m, "FooInt")
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   26 |         .def("foo", &Foo<int>::foo);
      |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~
ninja: build stopped: subcommand failed.

Reproducible example code

// foo.cpp
#include <iostream>
#include <pybind11/pybind11.h>


template <typename T>
struct Foo
{
    // foo() takes no parameters if T == int
    void foo() requires std::is_same_v<T, int>
    {
        std::cout << "int" << std::endl;
    }

    // foo() takes one parameter if T == double
    void foo(double d) requires std::is_same_v<T, double>
    {
        std::cout << "double" << std::endl;
    }
};

namespace py = pybind11;


PYBIND11_MODULE(bug, m)
{
    py::class_<Foo<int>>(m, "FooInt")
        .def("foo", &Foo<int>::foo);  // <-- compiler error
}

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

Not a regression

Start with the foo.cpp reproducer and the pybind11.h class_::def overloads shown in the compiler output. Investigate how the constrained C++ member-function overloads are handled, then verify that the Foo binding compiles and preserves the intended foo() interface.