[libcu++] Indirect indirect_binary invocable

[miscco]

The standard currently does only know indirectly_unary_invocable. However, there are a lot of algorithms like adjacent_difference where we want to make sure that we can invoke a functor by dereferencing two iterators.

Add this as an extension

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Overview

This pull request introduces indirectly_binary_invocable and indirectly_regular_binary_invocable concepts to libcu++, extending the iterator indirect-callability concept family. These new concepts enable checking whether a callable can be invoked with dereferenced pairs of iterators, filling a gap in the C++ standard library's concept definitions. This addresses requirements for algorithms like adjacent_difference that operate on pairs of elements.

Changes

Core Implementation

libcudacxx/include/cuda/std/__iterator/concepts.h

Added two new iterator indirect-callability concepts:

• indirectly_binary_invocable<_Fp, _It1, _It2>: Validates that a callable can be invoked with combinations of dereferenced iterator pairs
• indirectly_regular_binary_invocable<_Fp, _It1, _It2>: Similar to above, with regular invocation semantics

Both concepts require:

• Both iterators to be indirectly_readable
• The function object to be copy_constructible
• Comprehensive invocable/regular_invocable checks across combinations of iter_value_t, iter_reference_t, and iter_common_reference_t
• common_reference_with constraints linking invocation results across value/reference/cv variants

Implementation integrates with existing iterator concepts and predicates (matching the project's concept/macro fallbacks where applicable).

Test Coverage

libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_binary_invocable.compile.pass.cpp

Comprehensive compile-time validation featuring:

• GoodInvocable functor with overloads covering all required argument combinations
• 11 intentionally non-conforming invocables testing specific failure modes: iterator readability (gated by TEST_STD_VER), copy-constructibility, missing call expressions for value/reference/common-reference variants, and incompatible return-type common-reference relationships
• Additional tests for function pointer and function reference callables and several negative callable forms
• A trivial main returning 0

libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_regular_binary_invocable.compile.pass.cpp

Mirrors the binary invocable test structure with regular invocation semantics, validating both positive and negative cases for indirectly_regular_binary_invocable using equivalent test patterns and coverage.

Test Support

libcudacxx/test/support/indirectly_readable.h

Added IndirectlyReadable2<Token> template struct to support testing with pairs of distinct indirectly-readable iterators, mirroring the existing IndirectlyReadable<Token> pattern and providing common-reference test types used by the new tests.

Impact

• New concepts and tests are additive and confined to iterator concept machinery and test-support utilities.
• Lines added across implementation and tests: +~619 (implementation + tests + support), no deletions reported.
• Estimated review effort: Medium–High due to concept complexity and comprehensive negative-case coverage.

Walkthrough

suggestion: PR adds two binary iterator invocable concepts with parallel macro-based fallbacks, compile-pass tests for both positive and negative cases, and a small test-support helper for indirectly readable iterators.

Changes

Binary invocable concepts

Layer / File(s)	Summary
Binary invocable concept definitions libcudacxx/include/cuda/std/__iterator/concepts.h	Introduces indirectly_binary_invocable and indirectly_regular_binary_invocable concepts requiring both input iterators to be indirectly_readable, the function object to be copy_constructible, with invocable/regular_invocable checks across value/reference/common-reference forms and common_reference_with constraints on invoke_result_t variants. Provides both _CCCL_HAS_CONCEPTS and _CCCL_CONCEPT macro implementations.
Binary invocable compile-pass tests libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_binary_invocable.compile.pass.cpp	Validates indirectly_binary_invocable with positive static_assert for GoodInvocable and negative static_asserts covering iterator non-readability, deleted copy constructor, missing/deleted call operators for specific argument form combinations, and lack of common-reference among return types.
Regular binary invocable compile-pass tests libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_regular_binary_invocable.compile.pass.cpp	Validates indirectly_regular_binary_invocable with positive static_assert for GoodInvocable and negative static_asserts for the same constraint classes as the binary variant, including function-pointer and reference-callable cases.
Test support infrastructure libcudacxx/test/support/indirectly_readable.h	Added IndirectlyReadable2 template struct defining value_type and operator*() return type for binary iterator concept testing.

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🧹 Nitpick comments (1)

libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_binary_invocable.compile.pass.cpp (1)

37-43: ⚡ Quick win

suggestion: Add the symmetric unreadable-I2 negative assertion here. indirectly_binary_invocable requires both iterator operands to satisfy indirectly_readable, but this file only proves the failure when I1 is unreadable. A typo in either implementation path that re-checks I1 and forgets I2 would still pass this test. As per coding guidelines, libcudacxx/**/* reviews should focus on correctness, standard-library conformance, tests, and portability across supported CUDA toolchains.

Source: Coding guidelines

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📥 Commits

Reviewing files that changed from the base of the PR and between 82af814 and 4171c33.

📒 Files selected for processing (4)

• libcudacxx/include/cuda/std/__iterator/concepts.h
• libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_binary_invocable.compile.pass.cpp
• libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_regular_binary_invocable.compile.pass.cpp
• libcudacxx/test/support/indirectly_readable.h

🚧 Files skipped from review as they are similar to previous changes (3)

• libcudacxx/test/support/indirectly_readable.h
• libcudacxx/include/cuda/std/__iterator/concepts.h
• libcudacxx/test/libcudacxx/std/iterators/iterator.requirements/indirectcallable/indirectinvocable/indirectly_regular_binary_invocable.compile.pass.cpp

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[Jacobfaib]

Can we just combine these into just the _CCCL_CONCEPT form instead of duplicating? This is a massive concept to write out twice.