[libcu++] Change inplace_vector return type for try_meow methods

[davebayer]

This PR implements part of P3981R2 that changes inplace_vector return types for try_meow methods.

I've not updated the feature testing macro value, because there is another paper that we don't currently implement and they share the same macro value.

This is a source breaking change, but it's consistent what other standard libraries do when a feature is changed before the standard release.

[davebayer]

@miscco should I only include this header or rather full <cuda/std/optional>?

[coderabbitai]

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• libcudacxx/include/cuda/std/inplace_vector
• libcudacxx/test/libcudacxx/std/containers/sequences/inplace_vector/emplace.pass.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • inplace_vector methods try_emplace_back and try_push_back now return optional<reference> instead of raw pointers.
  • Capacity failures return nullopt instead of nullptr for improved type safety and consistent error handling.

Walkthrough

The PR changes inplace_vector's try_* insertion methods (try_emplace_back, try_push_back with const and rvalue variants) from returning raw pointers to returning optional<reference>. When the vector is full, these methods return nullopt; on success, they return an optional engaged with a reference to the inserted element. Both the general template and zero-capacity specialization are updated, along with test assertions.

Changes

Try_ API contract change*

Layer / File(s)	Summary
Try_ return type update to optional reference* libcudacxx/include/cuda/std/inplace_vector	Include optional_ref.h. Update try_emplace_back and both try_push_back overloads in the general template to return nullopt when full, otherwise an engaged optional<reference> from unchecked_emplace_back. Update zero-capacity specialization to always return nullopt.
Test assertions for optional-based try_ results* libcudacxx/test/libcudacxx/std/containers/sequences/inplace_vector/emplace.pass.cpp	Include <cuda/std/optional>. Update all try_* test assertions to expect optional<reference> return type, using res.value() for element access and assert(!res) for full-capacity failure cases.

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