Failure to normalize negative shifts

[baggepinnen]

The PR

• add component-based hybrid system test #2355

adds a test for discrete-time systems that is marked as @test_skip. The call to

prob = ODEProblem(ssys,  [model.plant.x => 0.0; model.controller.kp => 2.0; model.controller.ki => 2.0],  (0.0, Tf))

throws

ERROR: TypeError: in typeassert, expected Float64, got a value of type SymbolicUtils.BasicSymbolic{Float64}

and the problem is that one of the observed equations still contains a shift operator

x = 1.0 + Shift(t, -1)(0.0)

The problem is likely here

ModelingToolkit.jl/src/systems/systemstructure.jl

Line 321 in ddcace4

for s in (steps - 1):-1:1

where it's assumed that all shifts are positive