* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

This also disables catch for some other builds that happen to be in the same
matrix. But there are still plenty of builds running with DOWNLOAD_CATCH=ON.

Workaround for a windows-only download issue that started yesterday (2020-06-23):

-- Downloading catch v2.13.2...
CMake Error at tools/FindCatch.cmake:40 (message):
  Could not download
  https://github.com/philsquared/Catch/releases/download/v2.13.2/catch.hpp
Call Stack (most recent call first):
  tools/FindCatch.cmake:59 (_download_catch)
  tests/pure_cpp/CMakeLists.txt:1 (find_package)

* Updating and slightly enhancing instructions for running clang-tidy.

* Making documented commands identical to workflow commands. Adding comment in workflow file pointing to documentation.

More clang-tidy fixes. These escaped before because `-DDOWNLOAD_EIGEN=ON -DDOWNLOAD_CATCH=ON -DCMAKE_CXX_STANDARD=17`, as used in the GitHub Actions, were missing in the interactive run.

* Initial fixes

* Whoops

* Finish clang-tidy manual fixes

* Add two missing fixes

* Revert

* Update clang-tidy

* Try to fix unreachable code error

* Move nolint comment

* Apply missing fix

* Don't override clang-tidy config

* Does this fix clang-tidy?

* Make all clang-tidy errors visible

* Add comments about NOLINTs and remove a few

* Fix typo

* clang-tidy fixes (related to recent clang-tidy changes on master).

* More clang-tidy fixes.

* Enable and apply clang-tidy readability and misc fixes.

* Revert deprecated tester

* add space to tests/test_constants_and_functions.cpp

Example:
```
test_class_sh_unique_ptr_member.cpp:17:5: error: deleted member function should be public [modernize-use-equals-delete,-warnings-as-errors]
    pointee(const pointee &) = delete;
    ^
```

* Add more modernize clang-tidy checks

* Enable a few more

* Apply performance fixes from clang-tidy

* 2nd Round of Perf Optimizations

* 3rd round of fixes & handle false-positive

* Apply missing fix and clang-format

* Apply reviewer comment

* Apply clang-tidy performance fixes

* Fix bug introduced by double insert

* Revert all non-smart-holder changes

* Experiment using -DCMAKE_BUILD_TYPE=Debug for Centos 8.

* Moving comment because the current placement seems to mess up the cmake command.

* Using `echo > VAR_XXX` and `$(cat VAR_XXX)` trick to emulate using variables (https://github.com/actions/starter-workflows/issues/68#issuecomment-524937002).

Removing `std::get_deleter` `const_cast` to retest because the code has changed significantly. (#3045)

* Add const T to docstring generation.

* Change order.

* See if existing test triggers for a const type.

* Add tests.

* Fix test.

* Remove experiment.

* Reformat.

* More tests, checks run.

* Adding `test_fmt_desc_` prefix to new test functions.

* Using pytest.mark.parametrize to 1. condense test; 2. exercise all functions even if one fails; 3. be less platform-specific (e.g. C++ float is not necessarily float32).

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

* Drop constraints on casting of std::shared_ptr

std::shared_ptrs can be shared across python and C++ by design.

* Correctly report casting error

It is important to return an empty handle.
Simply returning None, would skip the error handling in
simple_collector / unpacking_collector, although a python exception is set.
A function call would then be processed with a (wrong) None argument!

* Return None for nullptr

* Revert "Drop constraints on casting of std::shared_ptr"

This reverts commit 7cf53ae8b4f99c0382845acf8bc63b64924dc891.