* use C++17 syntax to get rid of recursive template instantiations for concatenating type signatures (#4587)
* Apply descr.h `src_loc` change (smart_holder PR #4022) to code added with master PR #4587
* Add test_class_sh_property_non_owning to CMakeLists.txt (fixes oversight in PR #4586)
* Resolve clang-tidy errors.
* clang-tidy auto fix
---------
Co-authored-by: Konstantin Bespalov <kos5tya@yandex.ru>
* Add test_class_sh_property_non_owning.cpp,py
Failing:
```
__________________________________________________________ test_persistent_holder __________________________________________________________
def test_persistent_holder():
h = m.DataFieldsHolder(2)
> c = h.vec_at(0).core_fld
E RuntimeError: Non-owning holder (loaded_as_shared_ptr).
h = <pybind11_tests.class_sh_property_non_owning.DataFieldsHolder object at 0x7fabab516470>
test_class_sh_property_non_owning.py:6: RuntimeError
__________________________________________________________ test_temporary_holder ___________________________________________________________
def test_temporary_holder():
d = m.DataFieldsHolder(2).vec_at(1)
> c = d.core_fld
E RuntimeError: Non-owning holder (loaded_as_shared_ptr).
d = <pybind11_tests.class_sh_property_non_owning.DataField object at 0x7fabab548770>
test_class_sh_property_non_owning.py:13: RuntimeError
```
* Introduce `shared_ptr_from_python(responsible_parent)` and use in all `property_cpp_function`s with `const shared_ptr<T> &` arguments.
Tests are incomplete.
* Complete tests.
* Add comment for `smart_holder_type_caster_load<T>::shared_ptr_from_python`
