Throw an error if structural simplification is applied twice

Fixes #3012

Author: ChrisRackauckas

src/systems/systems.jl

@@ -3,6 +3,14 @@ function System(eqs::AbstractVector{<:Equation}, iv, args...; name = nothing,
     ODESystem(eqs, iv, args...; name, kw..., checks = false)
 end
 
+const REPEATED_SIMPLIFICATION_MESSAGE = "Structural simplification cannot be applied to a completed system. Double simplification is not allowed."
+
+struct RepeatedStructuralSimplificationError <: Exception end
+
+function Base.showerror(io::IO, e::RepeatedStructuralSimplificationError)
+    print(io, REPEATED_SIMPLIFICATION_MESSAGE)
+end
+
 """
 $(SIGNATURES)
 
@@ -21,6 +29,7 @@ function structural_simplify(
         sys::AbstractSystem, io = nothing; simplify = false, split = true,
         allow_symbolic = false, allow_parameter = true, conservative = false, fully_determined = true,
         kwargs...)
+    iscomplete(sys) && throw(RepeatedStructuralSimplificationError())
     newsys′ = __structural_simplify(sys, io; simplify,
         allow_symbolic, allow_parameter, conservative, fully_determined,
         kwargs...)

test/structural_transformation/errors.jl

@@ -0,0 +1,19 @@
+using ModelingToolkit
+using ModelingToolkit: t_nounits as t, D_nounits as D
+using Test
+
+@mtkmodel FOL begin
+    @parameters begin
+        τ = 3.0 # parameters
+    end
+    @variables begin
+        x(t) = 0.0 # dependent variables
+    end
+    @equations begin
+        D(x) ~ (1 - x) / τ
+    end
+end
+
+@named rc_model = ODESystem(rc_eqs, t; systems)
+sys = structural_simplify(rc_model)
+@test_throws ModelingToolkit.RepeatedStructuralSimplificationError structural_simplify(sys)

test/structural_transformation/runtests.jl

@@ -12,3 +12,6 @@ end
 @safetestset "Bareiss" begin
     include("bareiss.jl")
 end
+@safetestset "Errors" begin
+    include("errors.jl")
+end