Downgrade CI: Zygote (via LuxTestUtils) caps LogExpFunctions ≤0.3, conflicts with frozen 1.0.1 — transitive-dep wall, not floor-fixable

Summary

The Downgrade workflow fails at the julia-actions/julia-runtest re-resolve step (before any test runs) with an Unsatisfiable requirements detected for package Zygote error. This is a transitive-dependency downgrade wall, not a floor we can raise inside this repo.

Latest failing run: https://github.com/SciML/NeuralOperators.jl/actions/runs/27271571329 (job Downgrade / Downgrade Tests).

Note: the main env downgrade resolves fine (Successfully resolved minimal versions for .). The failure is only in the strict test-sandbox re-resolve.

Root cause (verified against the live General registry)

The conflict is LogExpFunctions pulled in two incompatible directions:

• The strict downgrade (allow_reresolve=false in the centralized downgrade.yml) freezes LogExpFunctions v1.0.1 from the main environment into the test sandbox (the modern Enzyme/Lux/NonlinearSolve stack floats it to 1.0.1; nothing in the main env forces it down).

• LuxTestUtils (test-only dep) transitively pulls in Zygote. Every Zygote version up to and including the latest 0.7.10 declares:

  ["0.6.41-0"]
  LogExpFunctions = "0.3.1-0.3"
  

  i.e. Zygote caps LogExpFunctions at 0.3 (resolver-expanded to ≤ 0.6.40). No Zygote release — not even the newest — accepts LogExpFunctions 1.x.

So the resolver sees LogExpFunctions = 1.0.1 (frozen, required by Enzyme ≥ 0.13.152) vs LogExpFunctions ≤ 0.6.40 (required by every available Zygote) → empty intersection → Unsatisfiable.

Exact resolver log:

ERROR: LoadError: Unsatisfiable requirements detected for package Zygote [e88e6eb3]:
 Zygote [e88e6eb3] log:
 ├─possible versions are: 0.1.0-0.7.10 or uninstalled
 ├─restricted by compatibility requirements with LogExpFunctions to versions: 0.1.0-0.6.40 or uninstalled
 │   └─restricted to versions 1.0.1 by an explicit requirement, leaving only versions: 1.0.1
 ...
 └─restricted by compatibility requirements with LuxTestUtils to versions: 0.6.70-0.7.10 — no versions left

Why this is not floor-fixable here

• Zygote is not a direct dependency of NeuralOperators (root or test/Project.toml); it is transitive via LuxTestUtils. There is no compat entry to raise.
• Zygote is already at its newest version (0.7.10) in the resolve, and even that newest version caps LogExpFunctions ≤ 0.3. Raising any NeuralOperators direct-dep floor cannot make Zygote accept LogExpFunctions 1.x.
• This is the same family as the fleet-wide ForwardDiff/LogExpFunctions strict-downgrade wall; here the capper is Zygote.

Suggested upstream fix (not in this repo)

One of:

• Zygote widens its LogExpFunctions compat to allow 1.x (its actual usage is almost certainly compatible); or
• LuxTestUtils stops transitively requiring Zygote on the downgrade path / loosens it.

Until an upstream release lands, this downgrade job is expected to be RED for a transitive-dependency reason; it does not reflect a NeuralOperators floor problem.

Separately (not this issue): master Core tests are also red

The regular (latest-versions) Tests / Core jobs are also failing on main, but for an unrelated reason — a genuine test assertion failure (test/layers/layers_testsetup.jl:41, test/models/fno_tests.jl:56), after a successful resolve where LogExpFunctions floats to 0.3.29. That is a separate problem from this downgrade resolution wall.

Start with the Downgrade workflow run and the centralized downgrade.yml setting allow_reresolve=false, then inspect the resolver output for Zygote, LuxTestUtils, and LogExpFunctions. Done requires an upstream compatibility or dependency change; the issue states that the fix is not in NeuralOperators.jl, so there is no in-repository patch to verify.