`deepcopy` leads to a failure in the initialization

[SebastianM-C]

Describe the bug 🐞

I noticed that with MTK@9.60 I can get an initialization failure if I deepcopy a particular system. I cannot reproduce it anymore with the current master, but I don't know if this issue was solve or it just no longer manifests.

Expected behavior

I would not expect to see any differences in behavior due to deepcopy.

Minimal Reproducible Example 👇

using ModelingToolkit, OrdinaryDiffEqRosenbrock
using ModelingToolkitStandardLibrary.Electrical
using ModelingToolkitStandardLibrary.Blocks: Sine
using ModelingToolkit: t_nounits as t

function create_model(; C₁=2.4, C₂=60.0, f=1.0)
    # V = 10.0
    @named resistor1 = Resistor(R=5.0)
    @named resistor2 = Resistor(R=2.0)
    @named capacitor1 = Capacitor(C=C₁)
    @named capacitor2 = Capacitor(C=C₂)
    @named source = Voltage()
    @named input_signal = Sine(frequency=f)
    @named ground = Ground()
    @named ampermeter = CurrentSensor()

    eqs = [connect(input_signal.output, source.V)
        connect(source.p, capacitor1.n, capacitor2.n)
        connect(source.n, resistor1.p, resistor2.p, ground.g)
        connect(resistor1.n, capacitor1.p, ampermeter.n)
        connect(resistor2.n, capacitor2.p, ampermeter.p)]

    @named circuit_model = ODESystem(eqs, t,
        systems=[
            resistor1, resistor2, capacitor1, capacitor2,
            source, input_signal, ground, ampermeter
        ])
end

model = create_model()
sys = structural_simplify(model)

@unpack ampermeter, capacitor2, capacitor1, resistor2 = model

ModelingToolkit.InitializationProblem(sys, 0, [capacitor2.v => 0.0,]) # works

ModelingToolkit.InitializationProblem(deepcopy(sys), 0, [capacitor2.v => 0.0,]) # ERROR: Initialization incomplete.

Error & Stacktrace ⚠️

ERROR: Initialization incomplete. Not all of the state variables of the
DAE system can be determined by the initialization. Missing
variables:

SymbolicUtils.BasicSymbolic{Real}[capacitor2₊v(t), capacitor1₊i(t)]

Stacktrace:
 [1] ModelingToolkit.InitializationProblem{…}(sys::ODESystem, t::Int64, u0map::Vector{…}, parammap::SciMLBase.NullParameters; guesses::Vector{…}, check_length::Bool, warn_initialize_determined::Bool, initialization_eqs::Vector{…}, fully_determined::Nothing, check_units::Bool, use_scc::Bool, kwargs::@Kwargs{})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1336
 [2] InitializationProblem
   @ ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1285 [inlined]
 [3] ModelingToolkit.InitializationProblem{true, SciMLBase.AutoSpecialize}(sys::ODESystem, t::Int64, u0map::Vector{Pair{…}})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1285
 [4] (ModelingToolkit.InitializationProblem{true})(::ODESystem, ::Int64, ::Vararg{Any}; kwargs::@Kwargs{})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1263
 [5] (ModelingToolkit.InitializationProblem{true})(::ODESystem, ::Int64, ::Vararg{Any})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1262
 [6] ModelingToolkit.InitializationProblem(::ODESystem, ::Int64, ::Vararg{Any}; kwargs::@Kwargs{})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1251
 [7] ModelingToolkit.InitializationProblem(::ODESystem, ::Int64, ::Vararg{Any})
   @ ModelingToolkit ~/.julia/packages/ModelingToolkit/iQ7So/src/systems/diffeqs/abstractodesystem.jl:1250
 [8] top-level scope
   @ ~/bug.jl:194
Some type information was truncated. Use `show(err)` to see complete types.

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

(dev) pkg> st -m ModelingToolkit ModelingToolkitStandardLibrary Symbolics SymbolicUtils
Status `~/dev/Manifest-v1.11.toml`
  [961ee093] ModelingToolkit v9.60.0
  [16a59e39] ModelingToolkitStandardLibrary v2.19.0
  [d1185830] SymbolicUtils v3.8.1
  [0c5d862f] Symbolics v6.22.1

• Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)

• Output of versioninfo()

julia> versioninfo()
Julia Version 1.11.2
Commit 5e9a32e7af2 (2024-12-01 20:02 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 32 × Intel(R) Core(TM) i9-14900K
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, alderlake)
Threads: 32 default, 0 interactive, 16 GC (on 32 virtual cores)
Environment:
  JULIA_EDITOR = code

[AayushSabharwal]

I'm not able to replicate this on master or MTK@9.60

[AayushSabharwal]

Okay yeah this is a hashconsing issue. It is fixed on the latest releases of Symbolics/SymbolicUtils