Workaround for #2682 and #2422 by simply clearing the TypeError

[bstaletic]

Description

As proposed in #2422 (comment), this PR is a possible solution for the assertion error in cpython debug mode.

Fixes#2682
Fixes#2422

Suggested changelog entry:

Fixed assertion error related to unhandled (later overwritten) exception in CPython 3.8 and 3.9 debug builds"

[YannickJadoul]

I think this might be a safer, more localized workaround than #2684, until we have a full fix for #2336 (which needs a little survey, to see in which places the casts are expected to return a nullptr handle and in which cases an exception is expected, given that it's such a central part of pybind11).

[albanD]

Thanks for the quick fix!

[YannickJadoul]

@albanD, so this also solves your problem, in combination with #2683?

[albanD]

Yes I can confirm that with the two fixes, the CI for debug cpython 3.8 and 3.9 now run without crashing.
Also 2.7 and 3.6 were not crashing before and are still working fine with the two fixes added.

[YannickJadoul]

Relevant links (except for just the three related issues #2682, #2422, and #2336):

• How [BUG] Python develop mode issues (3.8 and 3.9+) on macOS #2422 is just Casting from an unregistered type does not throw. #2336 in disguise (or a side-effect of the same cause, at least): [BUG] Python develop mode issues (3.8 and 3.9+) on macOS #2422 (comment)
• The seemingly obvious patch for Casting from an unregistered type does not throw. #2336: Casting from an unregistered type does not throw. #2336 (comment), which unfortunately doesn't work (because some internal parts of pybind11 have different expectations): Casting from an unregistered type does not throw. #2336 (comment)
• @bstaletic signaling [BUG] test_methods_and_attributes.py crash in cpython 3.9 debug build #2682 is a duplicate of [BUG] Python develop mode issues (3.8 and 3.9+) on macOS #2422: [BUG] test_methods_and_attributes.py crash in cpython 3.9 debug build #2682 (comment)
• Me arguing that @bstaletic's patch is a good intermediate solution, because Casting from an unregistered type does not throw. #2336 is messy: [BUG] test_methods_and_attributes.py crash in cpython 3.9 debug build #2682 (comment)

Note that there should be little to no danger not noticing this patch when #2336 will get patched (we have many tests that get triggered by changes that would fix #2336; see #2336 (comment)).

[henryiii]

I think this is safe for now.

[EricCousineau-TRI]

FWIW Ran into a similar issue in our fork, but a slightly different edge case (Eigen::AutoDiffScalar using dtype=object stuff):
RobotLocomotion#52

Just copied this solution there 😅