SDESystem with empty noise equations crashes in `check_units`

[isaacsas]

I assume for composition we'd want to allow SDESystems with no noise? In that case this errors out:

@parameters σ ρ β
@variables t x(t) y(t) z(t)
D = Differential(t)
eqs = [D(x) ~ σ*(y-x),
       D(y) ~ x*(ρ-z)-y,
       D(z) ~ x*y - β*z]
@named de = SDESystem(eqs,[],t,[x,y,z],[σ,ρ,β])

with error

ERROR: BoundsError: attempt to access 0-element Vector{Any} at index [1]
Stacktrace:
  [1] getindex
    @ ./array.jl:801 [inlined]
  [2] (::ModelingToolkit.var"#478#479"{String, Vector{Equation}, Vector{Any}})(idx::Int64)
    @ ModelingToolkit ./none:0
  [3] iterate
    @ ./generator.jl:47 [inlined]
  [4] collect(itr::Base.Generator{UnitRange{Int64}, ModelingToolkit.var"#478#479"{String, Vector{Equation}, Vector{Any}}})
    @ Base ./array.jl:678
  [5] #validate#477
    @ ~/.julia/packages/ModelingToolkit/9DDjU/src/systems/validation.jl:132 [inlined]
  [6] validate(eqs::Vector{Equation}, noise::Vector{Any})
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/9DDjU/src/systems/validation.jl:132
  [7] check_units(::Vector{Equation}, ::Vararg{Any, N} where N)
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/9DDjU/src/systems/validation.jl:138
  [8] SDESystem
    @ ~/.julia/packages/ModelingToolkit/9DDjU/src/systems/diffeqs/sdesystem.jl:94 [inlined]
  [9] SDESystem(deqs::Vector{Equation}, neqs::Vector{Any}, iv::Num, dvs::Vector{Num}, ps::Vector{Num}; controls::Vector{Num}, observed::Vector{Num}, systems::Vector{SDESystem}, default_u0::Dict{Any, Any}, default_p::Dict{Any, Any}, defaults::Dict{Any, Any}, name::Symbol, connection_type::Nothing, checks::Bool)
    @ ModelingToolkit ~/.julia/packages/ModelingToolkit/9DDjU/src/systems/diffeqs/sdesystem.jl:137
 [10] top-level scope
    @ REPL[69]:1

I'm not sure if the desired fix is to allow SDESystems without noise equations, or to allow ODE/Nonlinear/SDE systems to have sub-systems of different system types (which currently is not supported).

[lamorton]

I'm not sure why check_units is being called -- the guard conditional here all_dimensionless([dvs;ps;iv]) should be true. What version are you on?

It would be a pretty simple fix if we wanted to support this directly. We could just check the size of the noise eqs and don't pass them into check_units inside the SDE inner constructor if size is zero.

I think it would make more sense for this to be an ODESystem, but if there's already an expectation to allow SDESystems like this then we'll support it.

[isaacsas]

You are right, this is a classic case of my typing ]up ModelingToolkit earlier today and then not catching that it left me at 6.4.8... So it does indeed work in the unitless case. Sorry about that!

However, I do think it should be made to work with units too. Right now sub-systems are required to all have the same type. So any SDAEs or coupled ODE/SDE systems would have to be represented as SDESystems I think.

[lamorton]

Can we close this?