Downgrade CI tests have been failing since workflow was added

[ChrisRackauckas-Claude]

Description

The Downgrade and Downgrade Sublibraries CI workflows have been consistently failing since they were added in July 2025 (commit c7609b5).

Error Details

The root cause appears to be OrdinaryDiffEqNonlinearSolve failing to precompile when dependencies are downgraded to their minimum compat bounds:

Package OrdinaryDiffEqNonlinearSolve does not have SparseArrays in its dependencies

This error message is misleading because OrdinaryDiffEqNonlinearSolve DOES have SparseArrays in its [deps] section. The issue appears to be related to one of the transitive dependencies at its minimum version causing package loading issues.

Affected Workflows

• Downgrade.yml - Tests InterfaceI group with julia-downgrade-compat@v2
• DowngradeSublibraries.yml - Tests individual sublibraries with downgraded deps

Failing Sublibraries

Multiple sublibraries fail due to dependency on OrdinaryDiffEqNonlinearSolve:

• OrdinaryDiffEqRKN
• OrdinaryDiffEqExponentialRK
• OrdinaryDiffEqDefault
• OrdinaryDiffEqIMEXMultistep
• OrdinaryDiffEqStabilizedIRK
• OrdinaryDiffEqFIRK
• OrdinaryDiffEqAdamsBashforthMoulton
• OrdinaryDiffEqPDIRK
• OrdinaryDiffEqSDIRK
• OrdinaryDiffEqBDF
• OrdinaryDiffEqRosenbrock
• OrdinaryDiffEqLinear
• OrdinaryDiffEqTaylorSeries

Potential Fixes

Per the CI Health protocol, options include:

1. Option B: Bump [compat] - Increase minimum versions of dependencies causing issues
2. Option B4: Disable downgrade CI - Temporarily remove the failing tests from the CI matrix

Investigation Needed

• Identify which dependency at minimum version is causing the SparseArrays loading issue
• Test with incrementally higher minimum versions to find a working combination
• Consider if the minimum compat bounds are actually tested/supported

🤖 Generated with Claude Code