[llvm] [libc++abi] Handle null pointer-to-object: Issue #64593 (PR #68076)

[John McCall via llvm-commits]

https://github.com/rjmccall commented:

I'm... not sure it's okay to change v-tables in the `std::type_info` hierarchy like this.  In theory it's non-ABI and purely an internal implementation detail of the C++ runtime, but in practice there are some other subclasses out there, like in the ObjC runtime.

I know that the bot is telling you to reformat, but that is making it a lot more difficult to review this patch; please just ignore the bot or do the reformat in a separate PR.

Let me try to understand the basic problem here.  We have an exception that's dynamically a null pointer statically typed as `Base*`, and we're testing whether it's caught by `Derived*`.  This should succeed only if there is a unique public `Base` subobject in `Derived`.  Catching is built on top of the machinery for `dynamic_cast`, but the exception path can't be handled the same way it is for `dynamic_cast` for two reasons:
- First, the `dynamic_cast` operator on a null pointer just produces a null pointer unconditionally; it is not meaningful to ask whether the cast "succeeded".  That is not true with EH, because we have to decide whether we enter the `catch` block (or pass the EH filter).
- Second, in `dynamic_cast` we know both types statically and could therefore evaluate the unique-public-subclass condition statically if we needed to.  This is not possible with EH because the type relationship is dynamic.
So EH needs a way to check the has-unique-public-subclass condition without looking at actual types.  Makes sense.

https://github.com/llvm/llvm-project/pull/68076