bypass null-deref warnings

[nirandaperera]

Description

Fixes #7875

Enables -Wnull-dereference (with -Werror) in the libcudacxx test suite and suppresses the resulting GCC false-positive warnings in the __basic_any type-erasure implementation.

Why -Wnull-dereference?

-Wnull-dereference asks GCC to perform inter-procedural null-pointer flow analysis and warn (with -Werror, hard-fail) on any code path that can reach a dereference through a pointer that could be null. This catches a class of bugs that -Wall and -Wextra miss.

Why the suppressions?

GCC's analysis cannot see through __basic_any's type-erasure invariants. When __query_interface is inlined, GCC observes its return nullptr branch and concludes the result might be null at every call site, even those that are guarded by the design (e.g., __iunknown is always registered, virtual dispatch is only reached through a live object). This leads to false-positive errors in three locations:

File	Location	Root cause
virtcall.h	__virtcall — __vptr->__fn_(...)	__query_interface inlined; GCC sees potential nullptr
basic_any_value.h	reset() / type() — __query_interface(__iunknown())->...	Same: __iunknown lookup inlined, nullable path visible
rtti.h	__try_vptr_cast — rtti->__query_interface(...)	__query_interface(__iunknown()) inlined into slow-path cast

Each suppression is scoped as tightly as possible — a _CCCL_DIAG_PUSH / _CCCL_DIAG_SUPPRESS_GCC("-Wnull-dereference") / _CCCL_DIAG_POP bracket around only the offending statement(s). _CCCL_DIAG_SUPPRESS_GCC expands to a no-op on all non-GCC compilers.

Changes

• libcudacxx/test/CMakeLists.txt — adds -Wnull-dereference to headertest_warning_levels_device (NVIDIA/NVCC, Clang-as-device, and generic paths) and headertest_warning_levels_host (non-MSVC). MSVC paths are unchanged.
• libcudacxx/include/cuda/__utility/__basic_any/virtcall.h — suppresses false positive in __virtcall at the virtual dispatch call site.
• libcudacxx/include/cuda/__utility/__basic_any/basic_any_value.h — suppresses false positives in reset() and type() around the __query_interface(__iunknown()) dereference chains.
• libcudacxx/include/cuda/__utility/__basic_any/rtti.h — suppresses false positive in __try_vptr_cast slow-path (down-cast / cross-cast) where rtti is known non-null by the __iunknown registration invariant.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[pciolkosz]

Thanks for looking into it, it seems someone with cmake ownership needs to approve as well

[miscco]

Critical: some compilers have issues with warning suppression inside of class definitions. We should rather move the suppression outside to a single one at the top and one push at the bottom.

[nirandaperera]

@miscco what's the better practice here? Should I move the PUSH to the very top of the cpp file covering all methods? Or just cover these 2 methods?

Because, in the future, if some other warning needs to be suppressed, we should be able to see the existing warning suppressions.

[github-actions]

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