Fix method ambiguity in AbstractVectorOfArray similar() breaking QA on Julia 1.10

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Problem

GROUP=QA julia +1.10 --project -e 'using Pkg; Pkg.test()' fails on master (reproduced at 02c477c, "Run ode_gpu.jl in the GPU test group (#659)") with an Aqua method-ambiguity between Base.similar(::RecursiveArrayTools.AbstractVectorOfArray, ::Type{T}, ::Tuple{Union{Integer,Base.OneTo}, Vararg{...}}) (src/vector_of_array.jl:1157) and Base.similar(a::AbstractArray, ::Type{T}, dims::Tuple{Integer, Vararg{Integer}}) (abstractarray.jl:839). GROUP=QA on Julia 1.12 (and the 1 CI channel) passes.

CI does run QA on Julia 1.10: test/test_groups.toml has [QA] versions = ["lts", "1"], and "lts" currently resolves to 1.10. So this is a real CI break on master's lts QA lane, not a gap in coverage.

Root cause

Julia 1.10/1.11's Base defines three overlapping similar(::AbstractArray, ::Type, dims) fallbacks, including dims::Tuple{Integer, Vararg{Integer}} at abstractarray.jl:839. That particular fallback was removed in Julia 1.12 (methods(Base.similar, Tuple{AbstractArray,Type,Tuple}) only shows the NTuple{N,Int} and Tuple{Union{Integer,Base.OneTo},...} overloads there). RecursiveArrayTools defines similar(VA::AbstractVectorOfArray, ::Type{T}, dims::Tuple{Union{Integer,Base.OneTo}, Vararg{...}}), which is neither more nor less specific than Julia 1.10's Tuple{Integer, Vararg{Integer}} fallback once AbstractVectorOfArray <: AbstractArray (one dominates on the first argument, the other on the third) — hence the ambiguity, present only on Julia ≤ 1.11.

Bisect

• Introduced: cde5c35 ("Make AbstractVectorOfArray <: AbstractArray for proper interface compliance", PR BREAKING: Make AbstractVectorOfArray <: AbstractArray #547, merged 2026-04-01). Before this, AbstractVectorOfArray was not an AbstractArray, so Base's similar fallbacks never entered the dispatch intersection.
• Masked until: f3569c3 ("fix: resolve ArrayPartition ambiguity QA", PR fix: resolve ArrayPartition ambiguity QA #648, merged 2026-08-26) removed the blanket aqua_broken = (:ambiguities,) exemption. That PR's own verification fixed the copyto!/ldiv! ambiguities it found, but did not catch this one (version-dependent on Julia ≤ 1.11).
• Confirmed via git log -L 1157,1166:src/vector_of_array.jl, which traces the ambiguous method directly to cde5c35.

Fix

Adds similar(VA::AbstractVectorOfArray, ::Type{T}, dims::Tuple{Integer, Vararg{Integer}}), exactly the "possible fix" Aqua's own diagnostic suggests, pinning the Integer-only intersection so it dominates both the existing RAT method and Base's 1.10/1.11 fallback. This mirrors the same two-method split Base itself already uses (NTuple{N,Int} + Tuple{Integer,Vararg{Integer}}), so it does not introduce a new ambiguity. No Aqua exclusion used.

Verification

Before (git stash, Julia 1.10.12, private depot):

1 ambiguities found. To get a list, set `broken = false`.
Ambiguity #1
similar(VA::RecursiveArrayTools.AbstractVectorOfArray, ::Type{T}, dims::Tuple{Union{Integer, Base.OneTo}, Vararg{Union{Integer, Base.OneTo}}}) where T @ RecursiveArrayTools src/vector_of_array.jl:1157
similar(a::AbstractArray, ::Type{T}, dims::Tuple{Integer, Vararg{Integer}}) where T @ Base abstractarray.jl:839

Possible fix, define
  similar(::RecursiveArrayTools.AbstractVectorOfArray, ::Type{T}, ::Tuple{Integer, Vararg{Integer}}) where T

ERROR: LoadError: Some tests did not pass: 17 passed, 1 failed, 0 errored, 0 broken.
Test Summary:                                | Pass  Fail  Total   Time
Quality Assurance                            |   17     1     18  27.6s
  Method ambiguity                           |          1      1   4.4s

After (fix applied):

Test Summary:     | Pass  Total   Time
Quality Assurance |   18     18  18.0s

Also ran, all green:

• GROUP=QA, Julia 1.12.7: Quality Assurance | Pass 20 Total 20 (count differs from 1.10's 18 due to version-gated checks; both fully pass, before and after the fix).
• GROUP=Core, Julia 1.10.12: RecursiveArrayTools tests passed (all testsets, including VecOfArr Indexing/Interface Tests, pass).
• GROUP=Core, Julia 1.12.7: RecursiveArrayTools tests passed.
• Runic (Runic.main(["--diff","--check", "src/vector_of_array.jl"])): no diff, clean.
• typos src/vector_of_array.jl: clean.

Not verified

• GPU, Downstream, SymbolicIndexingInterface, AD, NoPre, and the 32-bit Core groups — not run locally; the change only touches a similar dispatch used by the ambiguity check and Core/QA groups above, but these were not re-run.
• Full GROUP=Everything.
• Did not re-run Aqua against the pre-#648 exemption state (i.e. confirming the ambiguity was silently present between #547 and #648); inferred from the code history (git log -L) rather than executed, since re-instantiating environments at each historical commit was not needed to identify the single-commit cause.

Risk assessment

• Risk: low
• Blast radius: adds one new, more-specific similar method for AbstractVectorOfArray; no existing method signature or behavior changes, no public API addition/removal, no version bump needed (internal dispatch fix only).
• Evidence: failing-before/passing-after QA output above; Core green on both Julia 1.10 and 1.12.
• Independent review: pending
• Merge: auto-merge candidate, pending independent review — small, mechanical, QA/Core green on both relevant Julia versions.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LPHREnnonfLg1VcE1EJovv

[ChrisRackauckas]

🤖 Automated comment from an AI agent running as @ChrisRackauckas — not written or reviewed by Chris.

Independent review (Devin CLI 3000.11.3, model fusion-claude-opus-5-5-high-sidekick-swe-2-medium): CHANGES, risk low.

Full review

VERDICT: CHANGES
RISK: low

The added method is correct. Running it confirms it removes the ambiguity on Julia 1.10 and 1.11. On 1.12 and 1.13 it is harmless: it returns the same results as master. Two problems remain. The PR adds no test that CI will actually run. The PR body also misdescribes what the bug is and where CI would catch it.

Blocking findings

1. No regression test that CI runs. The bug is a user-facing MethodError, not just an Aqua complaint. (src/vector_of_array.jl:1170; no file under test/ changed)

   • Confirmed by running scratch/probe.jl on the merge base 02c477c. With Julia 1.10.12 and 1.11.9, every call to similar(::AbstractVectorOfArray, T, dims) whose dims are non-Int integers throws. Examples: similar(VA, Float32, (Int32(2), 3)), (UInt8(2),), (big(2), 2), (true, 2). The error is MethodError: similar(::VectorOfArray{...}, ::Type{Float32}, ::Tuple{Int32, Int64}) is ambiguous. It happens for VectorOfArray (both 2-D and 3-D) and for DiffEqArray.
   • With the PR applied, all of those calls return Matrix{Float32} / Vector{Float32} of the right size on 1.10, 1.11, 1.12 and 1.13. On 1.12 and 1.13 the output is identical to the merge base's.
   • The PR's only before/after evidence is the Aqua ambiguity check in GROUP=QA. CI never runs QA on lts. SciML/.github scripts/compute_affected_sublibraries.jl pins the QA group to QA_VERSIONS = ["1"] regardless of what test_groups.toml says. The PR's own CI matrix (run 37807101121, detect job) contains only {"group":"QA","version":"1"}, and "1" now resolves to 1.13, where the ambiguity cannot occur.
   • So nothing in CI would catch a regression of this fix.
   • Required change: add a Core test. Core does run on lts. Put it, for example, in test/Core/interface_tests.jl, looping over a few non-Int dims tuples, e.g. (Int32(2), 3), (UInt8(2),), (Int32(2), Int32(3)). For each, assert similar(testva, Float32, dims) isa Array{Float32} and that size equals Int.(dims). On the merge base this test fails on Julia 1.10 with the MethodError above. Paste that failure and the post-fix pass into the PR body, as CLAUDE.md requires for bug fixes.

2. The PR body's central CI claim is false. (PR description, "Problem" section)

   • The body says "CI does run QA on Julia 1.10 … So this is a real CI break on master's lts QA lane". Confirmed by reading the CI matrix and the central script cited above: there is no QA lane on lts. On master, the QA lane runs on Julia "1" and passed (master CI run 36260267778).
   • The real impact is the user-facing MethodError on Julia 1.10/1.11 described in finding 1. Julia 1.10 is supported (julia = "1.10" compat).
   • Required change: rewrite the Problem / Risk sections to describe the actual bug: ambiguous similar with non-Int dims on Julia ≤ 1.11. Drop the CI-break claim. The "Merge: auto-merge candidate" rationale rests on that claim and should be reworded too.

Non-blocking findings

• The comment is too long. (src/vector_of_array.jl:1165-1169) Five of the ten added lines are comments, well above the <10% guideline in CLAUDE.md. The phrase "removed upstream in 1.12" is version-history narration. One line is enough, e.g. # Disambiguates against Base's Tuple{Integer,Vararg{Integer}} fallback on Julia ≤ 1.11.
• The Base claim checks out. Confirmed by running methods(similar, Tuple{AbstractArray,Type,Tuple}): 1.10.12 and 1.11.9 have the Tuple{Integer, Vararg{Integer}} fallback; 1.12.7 and 1.13.0 do not. 1.13 adds a Tuple{Union{Integer,Base.AbstractOneTo},...} fallback, and RAT's existing Union{Integer,Base.OneTo} method is strictly more specific than it, so 1.13 shows no new ambiguity.
• The new method adds no ambiguity of its own. Confirmed by running Test.detect_ambiguities(RecursiveArrayTools; recursive=true), filtered to RAT methods. It finds 1 ambiguity on base with 1.10/1.11, and 0 with the PR on 1.10, 1.11, 1.12 and 1.13. Dispatch also checks out: Tuple{Int,Int} still goes to the Tuple{Vararg{Int}} method, and Tuple{Int32,Int32} goes to the new method.
• Possible textual conflict with sibling PR fill!(::AbstractVectorOfArray, x): fill inner arrays directly, fixing a silent ragged bug #669. Its hunk starts at @@ -1167 in src/vector_of_array.jl, immediately after this PR's insertion point. Suspected from reading the hunk headers; I did not attempt a merge. No interface overlap with the other open RAT PRs (668, 671, 673 touch other regions).
• The bisect story is consistent with history, checked by reading only. cde5c35 adds abstract type AbstractVectorOfArray{T, N, A} <: AbstractArray{T, N}, and f3569c3 deletes aqua_broken = (:ambiguities,) from the test config. I did not run QA at historical commits.
• No new dependencies, exports or public API. There is no linked issue or tracking issue.

What I ran

All runs used TMPDIR set under the review directory. The merge base was extracted with git archive 02c477c into scratch/base, and the PR head with git archive HEAD into scratch/prcopy. Nothing was written inside repo/; git status there is clean.

• scratch/probe.jl on Julia 1.10.12, 1.11.9, 1.12.7 and 1.13.0, against both base and PR (logs in scratch/probe-{base,pr}-<ver>.log). It covers the Base fallback list, ambiguity detection, similar(A, Float32, dims) for VectorOfArray (2-D and 3-D) and DiffEqArray over 9 dims shapes, which dispatch, and the 2-argument similar(VA, dims). Results:
  • base on 1.10/1.11: 6 of 9 shapes throw an ambiguity MethodError for each type.
  • PR on all four versions: every shape returns a correctly sized Array.
  • On 1.11 the base probe stopped at the which(...) line with the same ambiguity error, after printing the same per-shape results as on 1.10.
• GROUP=QA julia +1.10 --project -e 'using Pkg; Pkg.test()'
  • base: Quality Assurance | 17 pass, 1 fail. The single ambiguity is exactly the one the PR quotes, and Aqua suggests the same "possible fix".
  • PR: QA | 18 pass, 18 total. This reproduces the PR's before/after.
• Runic (julia +1.12 --project=@runic -m Runic --check --diff src/vector_of_array.jl on the PR copy): clean. typos over the diff: clean.
• Read the PR's CI checks for head fc807f2: Core lts, 1, pre and x86, Downstream and SII all pass. The SciMLSensitivity Core1 downstream job fails; I did not investigate it, and it is unrelated to similar. I also read the CI detect matrix and the central QA clamping in SciML/.github scripts/compute_affected_sublibraries.jl@v1.

What I did not verify

• Did not run GROUP=Core locally on either Julia version. I relied on the passing CI Core jobs for SHA fc807f2 (status read, logs not inspected).
• Did not run QA on 1.12 or 1.13 locally. CI QA on "1" passed for this SHA.
• Did not investigate the failing SciMLSensitivity Core1 downstream job.
• Did not try merging with fill!(::AbstractVectorOfArray, x): fill inner arrays directly, fixing a silent ragged bug #669.
• Did not run QA at commits between cde5c35 and f3569c3.

Links

• PR under review: Fix method ambiguity in AbstractVectorOfArray similar() breaking QA on Julia 1.10 #678
• PR CI run (head fc807f2): https://github.com/SciML/RecursiveArrayTools.jl/actions/runs/37807101121
• Master CI run at merge base 02c477c: https://github.com/SciML/RecursiveArrayTools.jl/actions/runs/36260267778
• Sibling PR with an adjacent hunk: fill!(::AbstractVectorOfArray, x): fill inner arrays directly, fixing a silent ragged bug #669
• Central QA version clamp: https://github.com/SciML/.github/blob/v1/scripts/compute_affected_sublibraries.jl

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