sol[inlet.conc] does not work in flow connector

[cstjean]

Describe the bug 🐞

If I have a stream connector for a concentration, sol[inlet.concentration] should work. It doesn't, but it works for sol.outlet.concentration.

Minimal Reproducible Example 👇

using ModelingToolkit, DifferentialEquations
using ModelingToolkit: t_nounits as t, D_nounits as D

@connector SolutionPin begin
    flow(t), [connect=Flow]
    conc(t), [connect=Stream]
    dummy(t)
end

@mtkmodel Source begin
    @components begin
        outlet = SolutionPin()
    end
    @equations begin
        outlet.flow ~ -10
        outlet.conc ~ 0.3
    end
end

@mtkmodel Tank begin
    @components begin
        inlet = SolutionPin()
    end
    @variables begin
        product_mass(t) = 0
        conc(t)
    end
    @equations begin
        D(product_mass) ~ inlet.flow * instream(inlet.conc)
        conc ~ instream(inlet.conc)  # obviously not physically correct
    end
end

@mtkmodel Room begin
    @components begin
        src = Source()
        tank = Tank()
    end
    @equations begin
        connect(src.outlet, tank.inlet)
    end
end

@mtkbuild room = Room()
prob = ODEProblem(room, [], (0, 10.0), [])
sol = solve(prob)

sol[room.src.outlet.conc]  # works
sol[room.src.inlet.flow]  # works
sol[room.tank.inlet.conc] # ERROR: ArgumentError: tank₊inlet₊conc(t) is neither an observed nor an unknown 
sol[room.tank.conc] # works

Adding instream() does not work either (it shouldn't be required anyway).

[AayushSabharwal]

The problem is that tank.inlet.conc is not present in the simplified system, so MTK doesn't know how to compute it.

julia> room
Model room:
Equations (1):
  1 standard: see equations(room)
Unknowns (1): see unknowns(room)
  tank₊product_mass(t) [defaults to 0]
Observed (6): see observed(room)

julia> observed(room)
6-element Vector{Equation}:
 src₊outlet₊flow(t) ~ -10.0
 src₊outlet₊conc(t) ~ 0.3
 tank₊inlet₊dummy(t) ~ -0.0
 tank₊inlet₊flow(t) ~ -src₊outlet₊flow(t)
 tank₊conc(t) ~ src₊outlet₊conc(t)
 src₊outlet₊dummy(t) ~ tank₊inlet₊dummy(t)

julia> equations(room)
1-element Vector{Equation}:
 Differential(t)(tank₊product_mass(t)) ~ tank₊inlet₊flow(t)*src₊outlet₊conc(t)

[ChrisRackauckas]

Seems like a stream connector bug?

[AayushSabharwal]

Yeah likely. I'm looking into it.