fix: fix SCCNonlinearProblem reordering parameters

Author: AayushSabharwal

src/systems/nonlinear/nonlinearsystem.jl

@@ -792,7 +792,8 @@ function SciMLBase.SCCNonlinearProblem{iip}(sys::NonlinearSystem, u0map,
     new_eqs = eqs[reduce(vcat, eq_sccs)]
     @set! sys.unknowns = new_dvs
     @set! sys.eqs = new_eqs
-    sys = complete(sys)
+    @set! sys.index_cache = subset_unknowns_observed(
+        get_index_cache(sys), sys, new_dvs, getproperty.(obs, (:lhs,)))
     return SCCNonlinearProblem(subprobs, explicitfuns, p, true; sys)
 end
 

test/scc_nonlinear_problem.jl

@@ -263,3 +263,20 @@ end
     sol = solve(prob, NewtonRaphson())
     @test SciMLBase.successful_retcode(sol)
 end
+
+@testset "SCCNonlinearProblem retains parameter order" begin
+    @mtkbuild fullsys = NonlinearSystem(
+        [0 ~ x^3 * β + y^3 * ρ - σ, 0 ~ x^2 + 2x * y + y^2, 0 ~ z^2 - 4z + 4],
+        [x, y, z], [σ, β, ρ])
+
+    u0 = [x => 1.0,
+        y => 0.0,
+        z => 0.0]
+
+    p = [σ => 28.0,
+        ρ => 10.0,
+        β => 8 / 3]
+
+    sccprob = SCCNonlinearProblem(fullsys, u0, p)
+    @test isequal(parameters(fullsys), parameters(sccprob.f.sys))
+end