Discrete-time variables not found when forming InitializationProblem

[baggepinnen]

The following example fails with

julia> prob = ODEProblem(m, [m.delay.u(k-3)=>0, m.delay.u(k-2)=>0, m.delay.u(k-1)=>0], (0.0, 10.0))
ERROR: KeyError: key Shift(nothing, -3)(delay₊u(t)) not found
Stacktrace:
  [1] getindex(h::Dict{Any, Any}, key::SymbolicUtils.BasicSymbolic{Real})
    @ Base ./dict.jl:498
  [2] TearingState(sys::NonlinearSystem; quick_cancel::Bool, check::Bool)
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/systemstructure.jl:393
  [3] TearingState(sys::NonlinearSystem)
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/systemstructure.jl:250
  [4] __structural_simplify(sys::NonlinearSystem, io::Nothing; simplify::Bool, kwargs::@Kwargs{fully_determined::Bool})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/systems.jl:58
  [5] __structural_simplify
    @ ~/.julia/dev/ModelingToolkit/src/systems/systems.jl:55 [inlined]
  [6] structural_simplify(sys::NonlinearSystem, io::Nothing; simplify::Bool, split::Bool, kwargs::@Kwargs{…})
    @ ModelingToolkit ~/.julia/dev/ModelingToolkit/src/systems/systems.jl:22
  [7] structural_simplify (repeats 2 times)
    @ ~/.julia/dev/ModelingToolkit/src/systems/systems.jl:19 [inlined]
  [8] ModelingToolkit.InitializationProblem{…}(sys::ODESystem, t::Float64, u0map::Vector{…}, parammap::SciMLBase.NullParameters; guesses::Dict{…}, check_length::Bool, warn_initialize_determined::Bool, kwargs::@Kwargs{})

despite m.delay.u(k-3)=>0 being defined.

using ModelingToolkit
using ModelingToolkitStandardLibrary.Blocks
k = ShiftIndex()

@mtkmodel Del begin
    @extend u, y = siso = SISO()
    @structural_parameters begin
        n = 1
    end
    @equations begin
        y ~ u(k-n)
    end
end
@mtkmodel DiscreteInt begin
    @extend u, y = siso = SISO()
    @variables begin
        x(t) = 0.0, [description = "State of Integrator"]
    end
    @equations begin
        x(k) ~ x(k-1) + u(k-1)
        y ~ x
    end
end
@mtkmodel DelayModel begin
    @components begin
        delay = Del(n = 3)
        plant = DiscreteInt(x=0)
        input = Constant(k = 1)
    end
    @equations begin
        connect(input.output, delay.input)
        connect(delay.output, plant.input)
    end
end

@mtkbuild m = DelayModel()
prob = ODEProblem(m, [m.delay.u(k-3)=>0, m.delay.u(k-2)=>0, m.delay.u(k-1)=>0], (0.0, 10.0))

If the example is simplified to

@mtkmodel Del begin
    @extend u, y = siso = SISO()
    @structural_parameters begin
        n = 1
    end
    @equations begin
        y ~ u(k-n)
    end
end

@mtkmodel DelayModel begin
    @components begin
        delay = Del(n = 3)
        input = Constant(k = 1)
    end
    @equations begin
        connect(input.output, delay.input)
    end
end

@mtkbuild m = DelayModel()
prob = ODEProblem(m, [m.delay.u(k-3)=>0, m.delay.u(k-2)=>0, m.delay.u(k-1)=>0], (0.0, 10.0))

The initialization does work correctly, even though the simplification removed the system plant = DiscreteInt() rather than the system delay