[ChrisRackauckas]

🚧 UNREVIEWED — awaiting maintainer review. Filed by an AI agent; no human has reviewed this.
Harness: Codex CLI 0.162.1 · Model: gpt-6.1-sol
Session: 01a14bd6-bd53-7dd2-94c0-6f6613286f91

In-place VectorOfArray broadcasting slices array arguments along the destination's outer dimension. Lower-dimensional arguments broadcast across inner arrays, and singleton outer dimensions reuse their sole slice. Inner broadcasts use the destination axes for expansion, including immutable StaticArrays, StructArrays storage, and serial/Polyester FastBroadcast.

Fixes #680

Keep this draft out of merge consideration until a maintainer has reviewed it. No public signatures or dependencies change.

Reproduction and regression evidence

Reproduced on default-branch commit 214fb8d556b24581e65015bdeb340102aa33a76d (package version 4.5.5) and the latest registered release, 4.5.5, with Julia 1.12.7. Both produce [11.0 24.0; 12.0 25.0; 13.0 26.0] for the issue's vector-addition example instead of dense-array broadcasting's [11.0 14.0; 22.0 25.0; 33.0 36.0].

The new regression file compares against Base dense-array broadcasting for mutable vectors, SVectors, MVectors, StructArray storage, matrix inner arrays, zero-dimensional operands, singleton dimensions, and full-dimensional operands. It exercises serial and threaded FastBroadcast on SVector storage.

With the implementation fix stashed, running the regression file in a local environment that develops the checkout:

JULIA_NUM_THREADS=4 JULIA_NUM_PRECOMPILE_TASKS=4 timeout 3600 julia --project=tmp/main -e 'using RecursiveArrayTools, StaticArrays; v=VectorOfArray([SVector(1.,2.), SVector(3.,4.)]); f(v)=(v .= v .+ v); f(v); @show @allocated f(v); include("RecursiveArrayTools.jl/test/Core/voa_broadcast_dimensions.jl")'

Relevant output (stack traces omitted):

#= none:1 =# @allocated(f(v)) = 0
VectorOfArray broadcast dimensions: Test Failed
  Expression: Array(v) == expected
   Evaluated: [11.0 24.0; 12.0 25.0; 13.0 26.0] == [11.0 14.0; 22.0 25.0; 33.0 36.0]
DimensionMismatch: destination axes (Base.OneTo(3),) are not compatible with source axes ()
Test Summary:                      | Fail  Error  Total  Time
VectorOfArray broadcast dimensions |    1      1      2  2.0s

After restoring the fix, the same regression file:

JULIA_NUM_THREADS=4 JULIA_NUM_PRECOMPILE_TASKS=4 timeout 3600 julia --project=tmp/main -e 'include("RecursiveArrayTools.jl/test/Core/voa_broadcast_dimensions.jl")'

Test Summary:                      | Pass  Total  Time
VectorOfArray broadcast dimensions |  130    130  8.8s

Validation

All Julia commands use JULIA_NUM_THREADS=4, JULIA_NUM_PRECOMPILE_TASKS=4, and timeout 3600.

• GROUP=Core julia --project=RecursiveArrayTools.jl -e 'using Pkg; Pkg.test()': Testing RecursiveArrayTools tests passed; includes all 130 new checks and the existing zero-allocation assertions.
• GROUP=QA julia --project=RecursiveArrayTools.jl -e 'using Pkg; Pkg.test()': 20/20 passed.
• julia --project=@runic -m Runic --inplace on all six changed Julia files: passed.
• typos tmp/diff.patch on the complete diff, including the new test file, and git diff --check: passed.
• Docs build omitted: no docs, docstrings, or public API changes.
• Not run: Downstream, GPU, AD, NoPre, other Julia versions, and performance benchmarks. Independent review is pending.

Range-only FastBroadcast expressions can simplify to a range assignment before reaching these broadcast methods. A separate @.. v = x + 1 check with an SVector-backed destination and x = 10.:10.:30. fails on the unmodified default branch with setindex!(::SVector{3, Float64}, value, ::Int) is not defined. That assignment path is outside this fix; the tests cover lower-dimensional ranges through Base broadcasting.

Risk assessment

• Risk: medium-high (MH1: changes incorrect results and errors on the core VectorOfArray broadcasting lane).
• Blast radius: VectorOfArray and DiffEqArray broadcasting, including static inner arrays and the StructArrays/FastBroadcast extensions sharing the unpacking helper; no public signature changes.
• Evidence: reproduced on default branch and registered release; stashed-fix regression fails and restored-fix regression passes 130 checks; full Core and QA pass, including existing allocation checks. No downstream or GPU/AD validation.
• Independent review: pending (a different harness reviews it).
• Merge: needs human review — MH1 affects a widely used core entry point.

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🤖 Posted by an AI agent — harness: Codex CLI 0.162.1 · model: gpt-6.1-sol
Conversation: Codex exec session 01a14bd6-bd53-7dd2-94c0-6f6613286f91 (no session URL)