[libcu++] Implement tuple-like constructors for `tuple` by miscco · Pull Request #9059 · NVIDIA/cccl

miscco · 2026-05-18T18:34:54Z

This refactors the constructor constraints for cuda::std::tuple and implements the new tuple-like constructors

copy-pr-bot · 2026-05-18T18:34:57Z

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miscco · 2026-05-18T18:44:06Z

/ok to test

miscco · 2026-05-18T19:45:52Z

/ok to test

miscco · 2026-05-21T09:40:23Z

/ok to test

coderabbitai · 2026-05-21T09:48:20Z

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📝 Walkthrough

Walkthrough

Refactors CUDA libcudacxx tuple construction by introducing consteval-based constraint checkers and rebasing tuple-like trait detection, updates tuple/leaf/tuple_cat implementations, adds forward-declaration type traits, expands test coverage with pair/tuple conversion tests, and cleans up test directives.

Changes

Tuple construction and constraint machinery

Layer / File(s)	Summary
Forward-declaration type-detection traits `libcudacxx/include/cuda/std/__fwd/pair.h`, `libcudacxx/include/cuda/std/__fwd/subrange.h`, `libcudacxx/include/cuda/std/__fwd/tuple.h`	Adds `__is_cuda_std_pair`, `__is_cuda_std_ranges_subrange_v`, and `__is_cuda_std_tuple` variable templates for compile-time type detection of CUDA std types.
Tuple-like trait rebasing: __tuple_like_ext `libcudacxx/include/cuda/std/__tuple_dir/tuple_like.h`	Renames internal `__tuple_like_impl` to `__tuple_like_ext` and updates `__tuple_like` concept and `__pair_like` requires-clause to reference the new trait.
Constraint machinery: consteval checkers and trait structures `libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h`	Replaces predicate scaffolding with consteval construction selectors, comparison traits, and sentinel types; introduces `__select_constructor` enum, construction feasibility functions, and tuple-like/comparison detection helpers.
Tuple class constructors and comparison operators `libcudacxx/include/cuda/std/__tuple_dir/tuple.h`	Refactors default, allocator-arg, variadic copy/move, and tuple-like converting constructors to use checker-returned traits for enablement and `noexcept`; updates comparison operators; adds tuple `common_reference` and `common_type` specializations.
Tuple leaf implementation and tuple-like constructor tagging `libcudacxx/include/cuda/std/__tuple_dir/tuple_leaf.h`	Adds `__tuple_like_constructor_tag` for tag-dispatch, defaults leaf default constructor, and implements tag-dispatched tuple-like constructors; updates variadic constructor `noexcept` to fold over `is_nothrow_constructible_v`.
Helper trait updates: common_reference and deprecation suppression `libcudacxx/include/cuda/std/__type_traits/common_reference.h`, `libcudacxx/include/cuda/std/__type_traits/is_implicitly_default_constructible.h`	Extends `basic_common_reference` signature with trailing void parameter; wraps `is_implicitly_default_constructible` trait with deprecation suppression macros.
Tuple-cat and make_tuple_types dependent updates `libcudacxx/include/cuda/std/__tuple_dir/tuple_cat.h`, `libcudacxx/include/cuda/std/__tuple_dir/make_tuple_types.h`	Updates tuple_cat to use `__tuple_like_ext` for type detection; adds host-stdlib-guarded `__make_tuple_types_flat` specializations for `::std::array` and `::std::complex`.
Test support: copy/move probe types and tracing infrastructure `libcudacxx/test/support/copy_move_types.h`	Adds comprehensive test support header with copy/move construction probes (`MutableCopy`, `ConstCopy`, `MutableMove`, `ConstMove`), convertibility wrappers (`ConvertibleFrom`, `ExplicitConstructibleFrom`), tracing types (`TracedCopyMove`, `TracedAssignment`), tuple-reference conversion probes, and ImplicitlyConstructible detection; includes allocator-aware specializations.
Pair-to-tuple conversion tests `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_move_pair.pass.cpp`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/non_const_pair.pass.cpp`	Adds tests validating tuple construction from const and non-const pair rvalues with compile-time constraint checks (arity, constructibility, explicitness) and runtime tracing of constructor paths.
Tuple-to-tuple conversion tests `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_non_const_copy.pass.cpp`	Adds tests for const-rvalue and lvalue-reference tuple conversions verifying implicit/explicit rules, element-wise constructibility, arity matching, and traced constructor invocation; includes GCC-version-gated and MSVC-excluded scenarios.
Test directive cleanup and minor fixes `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.apply/make_from_tuple.pass.cpp`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.helper/`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.traits/trivially_copyable.pass.cpp`, `libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp`	Removes or narrows UNSUPPORTED/XFAIL directives, updates PR31384 with explicit tuple returns and host-vs-device expectations, adds [[maybe_unused]] annotation, and adds explicit defaulted copy constructor to NonTriviallyCopyAble.

Possibly related PRs

NVIDIA/cccl#9126: Refactors internal tuple/pair-like detection updating __pair_like to use __tuple_like and removing related scaffolding; foundational plumbing for this PR's constraint machinery.

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libcu++

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Actionable comments posted: 4

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📥 Commits

Reviewing files that changed from the base of the PR and between 589097d and 93d612c.

📒 Files selected for processing (16)

libcudacxx/include/cuda/std/__fwd/pair.h
libcudacxx/include/cuda/std/__fwd/subrange.h
libcudacxx/include/cuda/std/__fwd/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_cat.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_leaf.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_move_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_non_const_copy.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/non_const_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.traits/trivially_copyable.pass.cpp
libcudacxx/test/support/copy_move_types.h

💤 Files with no reviewable changes (1)

libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h

coderabbitai

🧹 Nitpick comments (2)

libcudacxx/include/cuda/std/__fwd/pair.h (1)

43-47: 💤 Low value

suggestion: Consider renaming to __is_cuda_std_pair_v for consistency with __is_tuple_of_iterator_references_v (line 40 of __fwd/tuple.h). The codebase has mixed patterns (__tuple_like_ext lacks _v, but iterator-references trait has it); standardizing on _v for boolean variable-template detection traits would improve uniformity.

libcudacxx/include/cuda/std/__fwd/tuple.h (1)

42-46: 💤 Low value

suggestion: Consider renaming to __is_cuda_std_tuple_v for consistency with __is_tuple_of_iterator_references_v at line 40 in this same file. Both are boolean variable-template detection traits; matching naming conventions within the same header improves readability.

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📥 Commits

Reviewing files that changed from the base of the PR and between 93d612c and f46bbc9.

📒 Files selected for processing (16)

libcudacxx/include/cuda/std/__fwd/pair.h
libcudacxx/include/cuda/std/__fwd/subrange.h
libcudacxx/include/cuda/std/__fwd/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_cat.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_leaf.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_move_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_non_const_copy.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/non_const_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.traits/trivially_copyable.pass.cpp
libcudacxx/test/support/copy_move_types.h

💤 Files with no reviewable changes (1)

libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h

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Actionable comments posted: 2

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📒 Files selected for processing (16)

libcudacxx/include/cuda/std/__fwd/pair.h
libcudacxx/include/cuda/std/__fwd/subrange.h
libcudacxx/include/cuda/std/__fwd/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_cat.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_leaf.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_move_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_non_const_copy.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/non_const_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.traits/trivially_copyable.pass.cpp
libcudacxx/test/support/copy_move_types.h

💤 Files with no reviewable changes (1)

libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h

✅ Files skipped from review due to trivial changes (1)

libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp

coderabbitai

🧹 Nitpick comments (2)

libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp (1)

116-131: ⚡ Quick win

suggestion: Lines 116-131 disable these constraint cases for every compiler via #if 0; gate them only for the compiler/version that actually segfaults so other toolchains keep this coverage.

libcudacxx/include/cuda/std/__fwd/subrange.h (1)

56-56: 💤 Low value

suggestion: Naming inconsistency with other detection traits. __is_cuda_std_ranges_subrange_v has a _v suffix, but __is_cuda_std_pair and __is_cuda_std_tuple do not. For internal consistency, either add _v to all three or remove it from this one.

Also applies to: 59-59

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📒 Files selected for processing (16)

libcudacxx/include/cuda/std/__fwd/pair.h
libcudacxx/include/cuda/std/__fwd/subrange.h
libcudacxx/include/cuda/std/__fwd/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_cat.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_leaf.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like.h
libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/PR31384.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/const_move_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_const_move.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/convert_non_const_copy.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.cnstr/non_const_pair.pass.cpp
libcudacxx/test/libcudacxx/std/utilities/tuple/tuple.tuple/tuple.traits/trivially_copyable.pass.cpp
libcudacxx/test/support/copy_move_types.h

💤 Files with no reviewable changes (1)

libcudacxx/include/cuda/std/__tuple_dir/tuple_like_ext.h

…n initializer

The test does fail with any other standard library, so just disable it

github-actions · 2026-06-01T15:54:39Z

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github-project-automation Bot added this to CCCL May 18, 2026

github-project-automation Bot moved this to Todo in CCCL May 18, 2026

cccl-authenticator-app Bot moved this from Todo to In Progress in CCCL May 18, 2026

miscco force-pushed the tupe_constraints_rewrite branch from 5a777c9 to fc262c0 Compare May 18, 2026 18:43

miscco force-pushed the tupe_constraints_rewrite branch 2 times, most recently from 305b7ec to c2fb71f Compare May 18, 2026 19:45

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miscco force-pushed the tupe_constraints_rewrite branch from c2fb71f to 93d612c Compare May 21, 2026 08:17

miscco marked this pull request as ready for review May 21, 2026 09:40

miscco requested a review from a team as a code owner May 21, 2026 09:40

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Comment thread libcudacxx/include/cuda/std/__tuple_dir/tuple.h Outdated

Comment thread libcudacxx/include/cuda/std/__tuple_dir/tuple.h

Comment thread libcudacxx/test/support/copy_move_types.h

Comment thread libcudacxx/test/support/copy_move_types.h

miscco force-pushed the tupe_constraints_rewrite branch from 93d612c to f46bbc9 Compare May 21, 2026 10:10

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miscco force-pushed the tupe_constraints_rewrite branch from f46bbc9 to 21aa1e3 Compare May 21, 2026 12:59

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Comment thread libcudacxx/include/cuda/std/__tuple_dir/tuple_constraints.h Outdated

Comment thread libcudacxx/test/support/copy_move_types.h

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Jacobfaib approved these changes May 21, 2026

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miscco force-pushed the tupe_constraints_rewrite branch from ce8a059 to f423ab2 Compare May 21, 2026 18:51

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miscco force-pushed the tupe_constraints_rewrite branch 3 times, most recently from 4ab569e to 57e3022 Compare May 22, 2026 08:52

miscco added 11 commits June 1, 2026 10:03

Drop removed type

0329178

Use shorter names

dbb012e

One more cleanup

f559bf2

Reenable test for GCC

742f0cb

Avoid old MSVC complaining about static constexpr variables needing a…

a77b013

…n initializer

Avoid warnings in implicitly default constructible

91ac121

Fix invalid nothrow determination

09313dc

Fix stly issues with tuple_leaf

259daa9

Address review comments

1714ca7

Drop invalid test.

82b52ba

The test does fail with any other standard library, so just disable it

Avoid pathological case with older GCC

40ce652

miscco force-pushed the tupe_constraints_rewrite branch from 619692c to 40ce652 Compare June 1, 2026 08:03

Also test allocator constructors

e0f8089

miscco force-pushed the tupe_constraints_rewrite branch 5 times, most recently from 2f21ed9 to 5fd347c Compare June 1, 2026 10:42

miscco added 3 commits June 1, 2026 12:58

Ensure we properly test the tuple like constructors

2d89e84

Use ugly get in tuple_leaf

3811dba

Suppress warnings about visibility

7a193ac

miscco force-pushed the tupe_constraints_rewrite branch from 5fd347c to 7a193ac Compare June 1, 2026 11:51

Explicitly rely on ADL when necessary

07ae089

davebayer approved these changes Jun 2, 2026

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miscco merged commit 9a8f7f6 into NVIDIA:main Jun 2, 2026
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miscco deleted the tupe_constraints_rewrite branch June 2, 2026 10:27

github-project-automation Bot moved this from In Review to Done in CCCL Jun 2, 2026

coderabbitai Bot mentioned this pull request Jun 4, 2026

[libcu++] Fix issues with new tuple constructors #9261

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andralex mentioned this pull request Jun 6, 2026

Constrain tuple_of_iterator_references -> tuple conversion on element convertibility (fixes GCC 9 build break) #9284

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