Test consistency check

Author: YingboMa

src/systems/systemstructure.jl

@@ -90,7 +90,7 @@ function initialize_system_structure(sys)
     eqs = copy(equations(sys))
     neqs = length(eqs)
     algeqs = trues(neqs)
-    dervaridxs = Int[]
+    dervaridxs = Set{Int}()
     var2idx = Dict{Any,Int}()
     symbolic_incidence = []
     fullvars = []
@@ -141,6 +141,7 @@ function initialize_system_structure(sys)
     end
 
     # sort `fullvars` such that the mass matrix is as diagonal as possible.
+    dervaridxs = collect(dervaridxs)
     sorted_fullvars = OrderedSet(fullvars[dervaridxs])
     for dervaridx in dervaridxs
         dervar = fullvars[dervaridx]

test/reduction.jl

@@ -212,3 +212,19 @@ A = reshape(1:N^2, N, N)
 eqs = xs .~ A * xs
 sys′ = NonlinearSystem(eqs, xs, [])
 sys = structural_simplify(sys′)
+
+# issue 958
+@parameters t k₁ k₂ k₋₁ E₀
+@variables E(t) C(t) S(t) P(t)
+D = Differential(t)
+
+equations = [
+             D(E) ~ k₋₁ * C - k₁ * E * S
+             D(C) ~ k₁ * E * S - k₋₁ * C - k₂ * C
+             D(S) ~ k₋₁ * C - k₁ * E * S
+             D(P) ~ k₂ * C
+             E₀ ~ E + C
+            ]
+
+@named sys = ODESystem(equations, t, [E, C, S, P], [k₁, k₂, k₋₁, E₀])
+@test_throws ModelingToolkit.InvalidSystemException structural_simplify(sys)