fix: fix structural_simplify/@mtkbuild for JumpSystem

Author: AayushSabharwal

src/systems/systems.jl

@@ -28,7 +28,7 @@ function structural_simplify(
     end
     if newsys isa ODESystem
         @set! newsys.parent = complete(sys; split)
-    else
+    elseif has_parent(newsys)
         @set! newsys.parent = complete(sys; split)
     end
     newsys = complete(newsys; split)
@@ -49,6 +49,11 @@ function structural_simplify(
         return newsys
     end
 end
+
+function __structural_simplify(sys::JumpSystem, args...; kwargs...)
+    return sys
+end
+
 function __structural_simplify(sys::AbstractSystem, io = nothing; simplify = false,
         kwargs...)
     sys = expand_connections(sys)

test/jumpsystem.jl

@@ -256,7 +256,17 @@ dp2 = DiscreteProblem(js2, u0, tspan)
 dp3 = DiscreteProblem(js3, u0, tspan, ps)
 dp4 = DiscreteProblem(js4, u0, tspan)
 
-jp1 = JumpProblem(js1, dp1, Direct())
-jp2 = JumpProblem(js2, dp2, Direct())
-jp3 = JumpProblem(js3, dp3, Direct())
-jp4 = JumpProblem(js4, dp4, Direct())
+@test_nowarn jp1 = JumpProblem(js1, dp1, Direct())
+@test_nowarn jp2 = JumpProblem(js2, dp2, Direct())
+@test_nowarn jp3 = JumpProblem(js3, dp3, Direct())
+@test_nowarn jp4 = JumpProblem(js4, dp4, Direct())
+
+# Ensure `structural_simplify` (and `@mtkbuild`) works on JumpSystem (by doing nothing)
+# Issue#2558
+@parameters k
+@variables X(t)
+rate = k
+affect = [X ~ X - 1]
+
+j1 = ConstantRateJump(k, [X ~ X - 1])
+@test_nowarn @mtkbuild js1 = JumpSystem([j1], t, [X], [k])