MTKBase: FunctionProperties extension exempting MTKParameters indexing

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas. Draft opened by an agent. Stacked on SciML/FunctionProperties.jl#62 — needs the FunctionProperties 0.1.7 release to resolve (compat is pinned to it). Companion to SciML/SciMLBase.jl#1413.

Summary

hasbranching (FunctionProperties.jl) decides whether a ReverseDiff tape can be compiled for an ODE RHS. For a split (default) MTK system the parameter object is an MTKParameters, and the generated RHS reads parameters through getindex(::MTKParameters, ::Int) — a @generated if idx == k … elseif … chain.

That branch is on the integer index, which is a literal constant at every real call site (so it constant-folds away in the compiled RHS), but the hasbranching recursion only sees the widened Int and reports it. Net effect: even a branch-free split-mode RHS such as ẋ = a·x is flagged as branching, needlessly disabling ReverseDiff compilation.

Change (lib/ModelingToolkitBase)

A weakdep extension marking the indexing as value-independent plumbing via the new is_leaf_sig hook from FunctionProperties#62:

FunctionProperties.is_leaf_sig(
    ::Type{<:Tuple{typeof(getindex), <:MTKParameters, Vararg}}) = true

Loaded only when FunctionProperties is present — no new hard dependency. Follows the existing ext/MTK*Ext.jl convention.

Tests

test/extensions/function_properties.jl (Extensions group): builds a split-mode ODEProblem, confirms prob.p isa MTKParameters, that the is_leaf_sig exemption is active for MTKParameters indexing (and does not over-reach to ordinary Vector), and that a function which only indexes the parameter container has no value-dependent branch. Verified locally (4/4) by loading the extension against a real MTKParameters. Runic clean.

Stack

• Detect value-independent branches: const-fold, splat-follow, is_leaf_sig FunctionProperties.jl#62 — is_leaf_sig + value-independent-branch detection (foundation).
• Add FunctionProperties extension: look through SciMLFunction wrappers SciMLBase.jl#1413 — unwrap AbstractSciMLFunction to its RHS.
• This PR — exempt MTKParameters integer indexing, completing the split-mode case.

Semver

Additive weakdep extension, non-breaking: MTKBase 1.48.1 → 1.48.2.

[AayushSabharwal]

The optimized IR should have all of this constant-folded, right? Why does FunctionProperties pass optimize = false?

[ChrisRackauckas-Claude]

Superseded. FunctionProperties #62 now folds the value-independent getindex(::MTKParameters, ::Int) branch generically via constant-propagation-aware analysis (the is_leaf_sig hook this extension used has been removed), so no MTK-side override is needed. Closing.