fix: add missing std::forward calls

[Boris-Rasin]

Two of the four cpp_function overloads are missing std::forward calls, which seems like a simple oversight.

Description

Suggested changelog entry:

add missing std::forward calls to some ``cpp_function`` overloads

[Skylion007]

This looks like an oversight from #2213 . Looks good to me. Ping @rwgk

[rwgk]

Hi @Boris-Rasin, thanks for fixing the oversight, looks good to me.
I'm guessing you noticed because something broke. Ideally we'd add corresponding new tests. Distilling tests from a known breakage is much easier than having to imagine them out of thin air. Is there a chance you could try distilling out unit tests? Alternatively, could you please post the error message(s) here?

[Boris-Rasin]

Hi @rwgk, ref qualified methods with rvalue reference parameters were broken:

struct RValueParam {
    void func1(std::string&&) {}
    void func2(std::string&&) const {}
    void func3(std::string&&) & {}
    void func4(std::string&&) const & {}
};

void init(pybind11::module& m)
{
  pybind11::class_<RValueParam>(m, "RValueParam")
    .def("func1", &RValueParam::func1) //ok
    .def("func2", &RValueParam::func2) //ok
    .def("func3", &RValueParam::func3) //error
    .def("func4", &RValueParam::func4) //error
  ;
}

// error message:
// pybind11/pybind11.h(118,74): error C2664: 'Return (std::string &&) &': cannot convert argument 1 from 'std::string' to 'std::string &&'

[rwgk]

Hi @rwgk, ref qualified methods with rvalue reference parameters were broken:

Thanks! It'd be great if you could add that to this PR, similar to what you see here (really straightforward):

https://github.com/pybind/pybind11/pull/2213/files

[rwgk]

I triggered the CI, although I'm not sure if you're still working on tests/test_methods_and_attributes.py, to prove that the methods are actually runnable. Anything trivial is good enough for this PR, e.g (untested).

std::size_t func1(std::string&& s) { return s.size(); }
...

assert obj.func1('boo') == 3
...

[rwgk]

Thanks a lot for adding the tests!
Holding off merging only to give @Skylion007 a chance to look at the tests, too.