Fix jacobian2W! DimensionMismatch for ScalarOperator mass matrix (#3915)

[singhharsh1708]

Fixes #3915.

Bug

jacobian2W!/jacobian2W special-case mass_matrix isa UniformScaling to skip the axes(mass_matrix) == axes(W) boundscheck and use the scalar directly. ScalarOperator is the same thing (λ·I) but isn't a UniformScaling, and reports axes(mm) == (), so it falls through to the boundscheck meant for full matrices and throws.

using OrdinaryDiffEqSDIRK, SciMLOperators
f = ODEFunction((du, u, p, t) -> (du .= -u); mass_matrix = ScalarOperator(1.0))
solve(ODEProblem(f, [1.0], (0.0, 1.0)), TRBDF2())
# DimensionMismatch: W: (Base.OneTo(1), Base.OneTo(1)), mass matrix: ()

IdentityOperator doesn't hit this since its axes happen to match W.

Fix

Treat ScalarOperator the same as UniformScaling in both jacobian2W! methods and jacobian2W, via _is_scalar_massmatrix/_scalar_massmatrix_λ. Follows #3814 → SciMLOperators#400 → #3837, the same operator-mass-matrix chain, one step further (this is the first place a ScalarOperator mass matrix reaches once the entry crash and get_differential_vars are both already fixed).

Tests

New scalar_operator_massmatrix_tests.jl: jacobian2W!/jacobian2W on ScalarOperator match the UniformScaling result directly. Also verified end-to-end (not in the test suite, checked locally): TRBDF2() on Lorenz with mass_matrix = ScalarOperator(2.0) now succeeds and matches mass_matrix = 2.0*I to the solver's tolerance.

OrdinaryDiffEqDifferentiation Core suite passes (24/24, including the new tests).

AI Disclosure

Claude assisted with this work.