fix: fix potential infinite recursion in simplification of OptimizationSystem

Author: AayushSabharwal

src/systems/optimization/optimizationsystem.jl

@@ -741,8 +741,9 @@ function structural_simplify(sys::OptimizationSystem; split = true, kwargs...)
     nlsys = NonlinearSystem(econs, unknowns(sys), parameters(sys); name = :___tmp_nlsystem)
     snlsys = structural_simplify(nlsys; fully_determined = false, kwargs...)
     obs = observed(snlsys)
-    subs = Dict(eq.lhs => eq.rhs for eq in observed(snlsys))
     seqs = equations(snlsys)
+    trueobs, _ = unhack_observed(obs, seqs)
+    subs = Dict(eq.lhs => eq.rhs for eq in trueobs)
     cons_simplified = similar(cons, length(icons) + length(seqs))
     for (i, eq) in enumerate(Iterators.flatten((seqs, icons)))
         cons_simplified[i] = fixpoint_sub(eq, subs)