Lsim fails for pure gain systems (D matrix only)

[matthewgcooper]

lsim() fails if it is passed a state space system with only a D matrix. Below is a minimum working example:

using ControlSystems

sys = pid_ss(1.0, 0)
lsim(sys, (_, _) -> [1.0], 0:0.1:1)

This produces the following error using ControlSystems v1.15.1

ERROR: LoadError: BoundsError: attempt to access 0-element Vector{Float64} at index [1]
Stacktrace:
  [1] throw_boundserror(A::Vector{Float64}, I::Tuple{Int64})
    @ Base ./essentials.jl:14
  [2] getindex
    @ ./essentials.jl:916 [inlined]
  [3] first
    @ ./abstractarray.jl:452 [inlined]
  [4] ode_interpolant
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/dense/generic_dense.jl:956 [inlined]
  [5] ode_interpolant
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/dense/generic_dense.jl:143 [inlined]
  [6] _savevalues!(integrator::OrdinaryDiffEqCore.ODEIntegrator{…}, force_save::Bool, reduce_size::Bool)
    @ OrdinaryDiffEqCore ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/integrators/integrator_utils.jl:110
  [7] savevalues! (repeats 2 times)
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/integrators/integrator_utils.jl:95 [inlined]
  [8] handle_callbacks!
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/integrators/integrator_utils.jl:406 [inlined]
  [9] _loopfooter!(integrator::OrdinaryDiffEqCore.ODEIntegrator{…})
    @ OrdinaryDiffEqCore ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/integrators/integrator_utils.jl:284
 [10] loopfooter!
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/integrators/integrator_utils.jl:248 [inlined]
 [11] solve!(integrator::OrdinaryDiffEqCore.ODEIntegrator{…})
    @ OrdinaryDiffEqCore ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/solve.jl:612
 [12] #__solve#62
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/solve.jl:7 [inlined]
 [13] __solve
    @ ~/.julia/packages/OrdinaryDiffEqCore/qzTLI/src/solve.jl:1 [inlined]
 [14] solve_call(_prob::SciMLBase.ODEProblem{…}, args::OrdinaryDiffEqTsit5.Tsit5{…}; merge_callbacks::Bool, kwargshandle::Nothing, kwargs::@Kwargs{…})
    @ DiffEqBase ~/.julia/packages/DiffEqBase/p82Yh/src/solve.jl:127
 [15] solve_up(prob::SciMLBase.ODEProblem{…}, sensealg::Nothing, u0::Vector{…}, p::Tuple{…}, args::OrdinaryDiffEqTsit5.Tsit5{…}; originator::SciMLBase.ChainRulesOriginator, kwargs::@Kwargs{…})
    @ DiffEqBase ~/.julia/packages/DiffEqBase/p82Yh/src/solve.jl:563
 [16] solve_up
    @ ~/.julia/packages/DiffEqBase/p82Yh/src/solve.jl:540 [inlined]
 [17] solve(prob::SciMLBase.ODEProblem{…}, args::OrdinaryDiffEqTsit5.Tsit5{…}; sensealg::Nothing, u0::Nothing, p::Nothing, wrap::Val{…}, kwargs::@Kwargs{…})
    @ DiffEqBase ~/.julia/packages/DiffEqBase/p82Yh/src/solve.jl:530
 [18] lsim(sys::StateSpace{…}, u::var"#11#12", t::StepRangeLen{…}; x0::Vector{…}, method::Symbol, dtmax::Float64, alg::OrdinaryDiffEqTsit5.Tsit5{…}, kwargs::@Kwargs{})
    @ ControlSystems ~/.julia/packages/ControlSystems/gkwy6/src/timeresp.jl:45
 [19] lsim(sys::StateSpace{Continuous, Float64}, u::Function, t::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64})
    @ ControlSystems ~/.julia/packages/ControlSystems/gkwy6/src/timeresp.jl:24
 [20] top-level scope
    @ ~/gnc_tools/TestDelete/test.jl:4
 [21] include(fname::String)
    @ Main ./sysimg.jl:38
 [22] top-level scope
    @ REPL[9]:1