Fix TypeError from nothing-holes in MTKParameters substitution (#4607)

[baggepinnen]

Fixes #4607.

Root cause

The issue's stacktrace (promote_symtype failing a ::DataType typeassert with Union{Nothing, Real}) is not caused by a variable with that symtype. Partially specifying an array variable (e.g. pinning a single component of a 3-vector connector flow) leaves COMMON_NOTHING = Const(nothing) holes for the remaining elements in the operating point (as_atomic_dict_with_defaults / write_possibly_indexed_array!); build_operating_point only filters values that are entirely nothing. When another operating-point entry's value expression references such a hole element, MTKParameters — which substituted against the raw operating point — folded Const(nothing) into arithmetic: promote_symtype(*, Real, Nothing) → promote_type = Union{Nothing, Real} → typeassert failure (or MethodError: *(::Int, ::Nothing) when all co-arguments are constant).

This only fires on the fully-determined initialization path because that is the one that builds an SCCNonlinearProblem for the initialization system, whose process_SciMLProblem → MTKParameters(initsys, op) evaluates all parameters (including Initials) against an op containing the partially-pinned arrays. The underdetermined sibling path instead hits #4237.

Changes

1. lib/ModelingToolkitBase/src/systems/parameter_buffer.jl: substitute through AtomicArrayDictSubstitutionWrapper (which exists precisely to keep holes symbolic, and is already used by every other substitution site against operating points — problem_utils.jl:348, problem_utils.jl:1599, sccnonlinearproblem.jl:564). Values of Initial parameters that consequently cannot be fully evaluated refer to variables not fixed by the operating point and are treated (elementwise) as unfixed, generalizing the existing val == args[1] → false special case; this matches how timevaring_initsys_process_op! turns non-constant pins into initialization equations rather than Initial values. Non-Initial parameters retain the informative Could not evaluate value of parameter error — which now also replaces the cryptic TypeError for genuinely missing values.

2. lib/ModelingToolkitBase/src/utils.jl: memoize the _check_operator_variables traversal. With the crash fixed, the repro proceeded into per-SCC generate_rhs and spun indefinitely (100% CPU, flat memory) in this check, which recursed into shared subexpressions repeatedly — exponential on the DAG-shaped expressions of large torn initialization systems.

3. src/problems/sccnonlinearproblem.jl: the MissingGuessValue.HashedRandom() branch (previously dead code, now reachable) called stable_eachindex on a plain Vector{Int} and referenced an undefined variable var. Fixed mirroring the equivalent branch in problem_utils.jl.

Validation

• New regression testset in lib/ModelingToolkitBase/test/mtkparameters.jl (the two crash modes now produce the informative error; unresolvable Initial values build; values referencing specified elements still fold).
• mtkparameters.jl, initial_values.jl, initializationsystem.jl (ModelingToolkitBase) and scc_nonlinear_problem.jl (ModelingToolkit) test files pass locally.
• The original FullCar repro (Dyad MultibodyComponents#suspension, private — not CI-able): pinning the chassis state (r_0, v_0, phi, phid) plus the four redundant chassis reaction torques (excited_suspension_*.chassis_frame.tau[2] => 0) previously threw the exact TypeError; with this PR the ODEProblem builds successfully. solve subsequently hits a SingularException from the chassis static indeterminacy — that is the separate, pre-existing Unhelpful initialization error #4237 and out of scope here.

🤖 Generated with Claude Code

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Benchmark Results (Julia vlts)

Time benchmarks

	master	6e724ba...	master / 6e724ba...
ODEProblem	14 ± 2.3 ms	11.6 ± 1.7 ms	1.21 ± 0.27
init	0.086 ± 0.031 ms	0.0809 ± 0.032 ms	1.06 ± 0.57
large_parameter_init/ODEProblem	0.0354 ± 0.0043 s	0.0365 ± 0.0058 s	0.967 ± 0.19
large_parameter_init/init	0.0908 ± 0.019 ms	0.096 ± 0.027 ms	0.945 ± 0.33
mtkcompile	15.1 ± 3 ms	13.2 ± 1.7 ms	1.15 ± 0.27
sparse_analytical_jacobian/ODEProblem	0.0404 ± 0.0038 s	0.0415 ± 0.0046 s	0.973 ± 0.14
sparse_analytical_jacobian/f_iip	1.89 ± 0.56 μs	1.47 ± 0.49 μs	1.28 ± 0.57
sparse_analytical_jacobian/f_oop	0.37 ± 0.015 ms	0.364 ± 0.014 ms	1.02 ± 0.055
time_to_load	6.9 ± 0.21 s	6.22 ± 0.32 s	1.11 ± 0.065

Memory benchmarks

	master	6e724ba...	master / 6e724ba...
ODEProblem	0.0733 M allocs: 3.14 MB	0.0734 M allocs: 3.14 MB	1
init	0.384 k allocs: 0.065 MB	0.384 k allocs: 0.065 MB	1
large_parameter_init/ODEProblem	0.298 M allocs: 10.3 MB	0.301 M allocs: 10.4 MB	0.987
large_parameter_init/init	0.551 k allocs: 0.166 MB	0.551 k allocs: 0.166 MB	1
mtkcompile	0.093 M allocs: 3.94 MB	0.093 M allocs: 3.94 MB	1
sparse_analytical_jacobian/ODEProblem	0.261 M allocs: 9.23 MB	0.261 M allocs: 9.23 MB	1
sparse_analytical_jacobian/f_iip	0 allocs: 0 B	0 allocs: 0 B
sparse_analytical_jacobian/f_oop	0.634 k allocs: 19.6 kB	0.634 k allocs: 19.6 kB	1
time_to_load	0.153 k allocs: 14.6 kB	0.153 k allocs: 14.6 kB	1

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	6e724ba...	master / 6e724ba...
ODEProblem	10.2 ± 1.1 ms	9.89 ± 0.61 ms	1.04 ± 0.12
init	0.053 ± 0.008 ms	0.0519 ± 0.0076 ms	1.02 ± 0.21
large_parameter_init/ODEProblem	0.0318 ± 0.0052 s	0.0322 ± 0.0045 s	0.988 ± 0.21
large_parameter_init/init	0.0647 ± 0.0042 ms	0.0655 ± 0.004 ms	0.987 ± 0.089
mtkcompile	10.6 ± 1.2 ms	9.88 ± 0.61 ms	1.07 ± 0.14
sparse_analytical_jacobian/ODEProblem	0.0355 ± 0.0032 s	0.0332 ± 0.0032 s	1.07 ± 0.14
sparse_analytical_jacobian/f_iip	0.1 ± 0.01 μs	0.09 ± 0.01 μs	1.11 ± 0.17
sparse_analytical_jacobian/f_oop	0.12 ± 0.0046 ms	0.122 ± 0.0051 ms	0.989 ± 0.056
time_to_load	5.91 ± 0.019 s	6.04 ± 0.038 s	0.978 ± 0.0069

Memory benchmarks

	master	6e724ba...	master / 6e724ba...
ODEProblem	0.0439 M allocs: 2.32 MB	0.0439 M allocs: 2.32 MB	0.999
init	0.345 k allocs: 0.0447 MB	0.345 k allocs: 0.0447 MB	1
large_parameter_init/ODEProblem	0.262 M allocs: 9.91 MB	0.265 M allocs: 10.1 MB	0.977
large_parameter_init/init	0.685 k allocs: 0.144 MB	0.685 k allocs: 0.144 MB	1
mtkcompile	0.0672 M allocs: 2.85 MB	0.0672 M allocs: 2.84 MB	1
sparse_analytical_jacobian/ODEProblem	0.201 M allocs: 7.34 MB	0.201 M allocs: 7.34 MB	1
sparse_analytical_jacobian/f_iip	0 allocs: 0 B	0 allocs: 0 B
sparse_analytical_jacobian/f_oop	0.848 k allocs: 27 kB	0.848 k allocs: 27 kB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1