[libcu++] Reformulate tuple constraints to reduce memory consumption

[miscco]

We are experiencing memory consumption issues recently. One of the major changes was a rewrite of tuple constraints to support the new tuple like constructors.

This refactors the machinery to be closer to what we had previously in hopes it reduces the memory consumption

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Reviewing files that changed from the base of the PR and between 22ac0e7 and 9b4360e.

📒 Files selected for processing (1)

• libcudacxx/include/cuda/std/__tuple_dir/tuple.h

🚧 Files skipped from review as they are similar to previous changes (1)

• libcudacxx/include/cuda/std/__tuple_dir/tuple.h

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Improved how cuda::std::tuple and cuda::std::pair decide which constructors and assignment operators are enabled or deleted.
  • Centralized tuple constraint-based overload selection for default/copy/move, forwarding/variadic, and tuple-like operations, preserving the same enablement and deleted/explicit behavior.
  • Updated tuple comparison constraints so equality and ordering operators compute comparability/no-throw directly from the centralized selectors, with updated operator template signatures.

Walkthrough

Refactors cuda::std::tuple and pair to route constructor, assignment, and comparison constraint selection through __tuple_constraints<_Types...>, replacing prior free-standing helpers and _v trait forms.

Changes

Constraint helper consolidation

Layer / File(s)	Summary
New __tuple_constraints struct definition libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h	Adds __tuple_constraints<_Types...> static selection functions for constructor, tuple-like, assignment, and comparison constraints.
tuple constructor constraint wiring libcudacxx/include/cuda/std/__tuple_dir/tuple.h	Adds a local __constraints alias in tuple<_Tp...> and rewrites constructor and tuple-like construction trait selection to use __tuple_constraints members.
tuple assignment and comparison wiring libcudacxx/include/cuda/std/__tuple_dir/tuple.h	Updates tuple<_Tp...> assignment trait selection and rewrites equality and ordering operators to use the new comparison booleans and helper.
pair constructor constraint wiring libcudacxx/include/cuda/std/__utility/pair.h	Adds a __constraints alias in __pair_base and pair, and rewrites default, copy, and variadic forwarding constructor trait selection to use __tuple_constraints members.

Possibly related PRs

• NVIDIA/cccl#9683: Both modify tuple_constraints.h constructor selection logic, including the variadic-constructible selection machinery.

Suggested reviewers: jrhemstad, Jacobfaib

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Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 4c466d95-6165-453f-b6a7-3687fce8e2cf

📥 Commits

Reviewing files that changed from the base of the PR and between 174a652 and 337de0d.

📒 Files selected for processing (3)

• libcudacxx/include/cuda/std/__tuple_dir/tuple.h
• libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h
• libcudacxx/include/cuda/std/__utility/pair.h

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