[libcu++] Fix __detectably_invalid value returned during constant evaluation

[davebayer]

In host/device mdspans, we check whether the pointer address corresponds with the type in __detectably_invalid accessor method. However, we have no way to find out during constant evaluation. Unfortunately we returned true (always invalid) instead of false (always valid) in this case. This PR fixes that.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Corrected accessor validation logic so validity checks yield accurate boolean results across compilation contexts, avoiding spurious device/host usage errors.
  • Improves consistency of host/device memory operation checks without changing public interfaces.

suggestion:

Walkthrough

Three accessor implementations adjust consteval handling: __host_accessor::access wraps a device-target verification inside _CCCL_IF_NOT_CONSTEVAL_DEFAULT, and the __detectably_invalid fallback return value changes from true to false in __host_accessor, __device_accessor, and __managed_accessor.

Changes

Accessor consteval and fallback changes

Layer / File(s)	Summary
Host access: device-target check guarded by not-consteval libcudacxx/include/cuda/__mdspan/host_device_accessor.h	__host_accessor::access (lines ~199-202) now emits the NV_IF_TARGET(NV_IS_DEVICE, ...) verification only inside an _CCCL_IF_NOT_CONSTEVAL_DEFAULT block.
Consteval fallback correction in __detectably_invalid libcudacxx/include/cuda/__mdspan/host_device_accessor.h	__host_accessor::__detectably_invalid (line ~223), __device_accessor::__detectably_invalid (line ~358), and __managed_accessor::__detectably_invalid (line ~478) change their _CCCL_IF_NOT_CONSTEVAL_DEFAULT fallback return value from true to false.

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[coderabbitai]

🧹 Nitpick comments (2)

libcudacxx/include/cuda/__mdspan/host_device_accessor.h (2)

199-202: ⚡ Quick win

suggestion: add a regression test that exercises __host_accessor::access in constant evaluation and non-constant-evaluation device compilation paths, so the new _CCCL_IF_NOT_CONSTEVAL_DEFAULT guard behavior is pinned.

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223-223: ⚡ Quick win

suggestion: add constexpr-focused regression tests for __detectably_invalid in host/device/managed accessors to lock the new return false fallback and prevent behavior drift.

As per coding guidelines: "libcudacxx/**/*: Focus on correctness, ABI/API stability, standard-library conformance, host/device availability, NVRTC compatibility, supported C++ standards, tests, and portability across supported CUDA toolchains."

Also applies to: 358-358, 478-478

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between b832c9f and f8168ba.

📒 Files selected for processing (1)

• libcudacxx/include/cuda/__mdspan/host_device_accessor.h