[libcu++] Implement P3798R1 The unexpected in std::expected

[davebayer]

This PR implements P3798R1 and backports it to C++17.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Run ID: 7a57d342-97b2-4311-a221-18bcb4732a81

📥 Commits

Reviewing files that changed from the base of the PR and between 332a726 and ae15777.

📒 Files selected for processing (4)

• libcudacxx/include/cuda/std/__expected/expected.h
• libcudacxx/include/cuda/std/version
• libcudacxx/test/libcudacxx/std/utilities/expected/expected.expected/observers/has_error.pass.cpp
• libcudacxx/test/libcudacxx/std/utilities/expected/expected.void/observers/has_error.pass.cpp

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added a new has_error() observer for cuda::std::expected and cuda::std::expected<void, E> to quickly check whether a value is absent.
  • Updated the library feature version to reflect the new expected capability.

• Tests

  • Added coverage for has_error() in both value and void specializations, including constexpr and noexcept behavior.

Walkthrough

Adds a has_error() observer method to cuda::std::expected<T, E> and its expected<void, E> specialization, returning the negation of has_value(). Updates the __cccl_lib_expected feature-test macro version. Adds corresponding test files validating behavior and noexcept qualification for both specializations.

Changes

has_error() observer addition

Layer / File(s)	Summary
has_error() implementation and version macro libcudacxx/include/cuda/std/__expected/expected.h, libcudacxx/include/cuda/std/version	Adds constexpr bool has_error() const noexcept returning !__has_val_ to both expected<T, E> and expected<void, E>; bumps __cccl_lib_expected from 202211L to 202606L.
expected<T,E> has_error() tests libcudacxx/test/.../expected.expected/observers/has_error.pass.cpp	New test validates has_error() runtime behavior for error/value states, checks noexcept qualification via a detection trait, and optionally runs a constexpr static_assert.
expected<void,E> has_error() tests libcudacxx/test/.../expected.void/observers/has_error.pass.cpp	New test with headers/includes validates has_error() noexcept qualification and runtime/constexpr behavior for expected<void, int> in error and default states.

Sequence Diagram(s)

Not applicable — changes are simple accessor additions and test files without multi-component interaction flows.

Estimated code review effort: Low

Suggested labels: libcudacxx, feature, test

Suggested reviewers: none identified from provided data

important: verify has_error() is documented/added to the public spec/changelog for expected, not just the feature-test macro bump.

suggestion: consider adding a symmetric noexcept static_assert test for the general expected<T,E> case matching the void specialization's Foo trait check, if not already covered.

🐰 A hare hopped through expected's den,
found has_value's shadow, named it again —
"has_error," it chirped, noexcept and neat,
tests confirm the trick is complete.

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[Jacobfaib]

Suggested change

	_CCCL_API constexpr bool has_error() const noexcept
	[[nodiscard]] _CCCL_API constexpr bool has_error() const noexcept

[davebayer]

I want to make a separate PR adding [[nodiscard]] to expected

[Jacobfaib]

Suggested change

	_CCCL_API constexpr bool has_error() const noexcept
	[[nodiscard]] _CCCL_API constexpr bool has_error() const noexcept

[github-actions]

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