BoundsError with DAE system when defining t as a variable instead of parameter

[valentinsulzer]

Defining t with the variables instead of with the parameters works fine if making an ODE model, but leads to errors if some equations are algebraic.
Should an error be raised if t is defined as a variable instead of as a parameter?

Works

@parameters t
@variables x1(t) x2(t)
@derivatives D'~t

eqs = [
   D(x1) ~ -x1,
   0 ~ x1 - x2,
]
sys = ODESystem(eqs, t)

u0 = [
   x1 => 1.0,
   x2 => 1.0,
]
prob = ODEProblem(sys,u0,tspan)
sol = solve(prob,Rodas5())

Fails

@variables t x1(t) x2(t)
@derivatives D'~t

eqs = [
   D(x1) ~ -x1,
   0 ~ x1 - x2,
]
sys = ODESystem(eqs, t)

u0 = [
   x1 => 1.0,
   x2 => 1.0,
]
prob = ODEProblem(sys,u0,tspan)
sol = solve(prob,Rodas5())

with message

BoundsError: attempt to access 2-element Array{Float64,1} at index [3]

Stacktrace:
 [1] setindex! at ./array.jl:847 [inlined]
 [2] varmap_to_vars(::Array{Pair{Operation,Float64},1}, ::Array{Variable,1}) at /Users/vsulzer/.julia/packages/ModelingToolkit/4JKjD/src/variables.jl:254
 [3] ODEProblem{true,tType,isinplace,P,F,K,PT} where PT where K where F where P where isinplace where tType(::ODESystem, ::Array{Pair{Operation,Float64},1}, ::Tuple{Float64,Float64}, ::DiffEqBase.NullParameters; version::Nothing, tgrad::Bool, jac::Bool, checkbounds::Bool, sparse::Bool, simplify::Bool, linenumbers::Bool, parallel::ModelingToolkit.SerialForm, eval_expression::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/vsulzer/.julia/packages/ModelingToolkit/4JKjD/src/systems/diffeqs/abstractodesystem.jl:260
 [4] ODEProblem at /Users/vsulzer/.julia/packages/ModelingToolkit/4JKjD/src/systems/diffeqs/abstractodesystem.jl:258 [inlined] (repeats 2 times)
 [5] ODEProblem(::ODESystem, ::Array{Pair{Operation,Float64},1}, ::Vararg{Any,N} where N; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/vsulzer/.julia/packages/ModelingToolkit/4JKjD/src/systems/diffeqs/abstractodesystem.jl:231
 [6] ODEProblem(::ODESystem, ::Array{Pair{Operation,Float64},1}, ::Vararg{Any,N} where N) at /Users/vsulzer/.julia/packages/ModelingToolkit/4JKjD/src/systems/diffeqs/abstractodesystem.jl:231
 [7] top-level scope at In[81]:15
 [8] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091

[ChrisRackauckas]

We need to revisit this whole part of the interface after #518 merges @shashi @YingboMa

[YingboMa]

Using @variables to define independent variable is not expected to always work.

[ChrisRackauckas]

Is there a reason why discovery is hard in this case? Seems odd to have to label it a parameter.

[YingboMa]

This particular issue isn't hard to fix, but I think it could break some assumptions in passes.

[ChrisRackauckas]

Which passes assume that t is a Parameter?

[YingboMa]

Oh, I did a grep on the source. There seems to be none.