Symbolic-array system bug in counting equations and variables

[baggepinnen]

MWE

@mtkmodel Foo begin
    @variables begin
        x(t)[1:2]
    end
    @equations begin
        D.(x) ~ .-x
    end
end
@named foo = Foo()
foo = complete(foo)
prob = ODEProblem(foo, Dict(foo.x => [1.0, 1]), (0.0, 1.0))
sol = solve(prob, Rodas4())

julia> prob = ODEProblem(foo, Dict(foo.x => [1.0, 1]), (0.0, 1.0))
ERROR: ArgumentError: Equations (1), unknowns (2), and initial conditions (2) are of different lengths. To allow a different number of equations than unknowns use kwarg check_length=false.

[baggepinnen]

If I instead broadcast the equation, I get another error

@mtkmodel Foo begin
    @variables begin
        x(t)[1:2]
    end
    @equations begin
        D.(x) .~ .-x
    end
end
@named foo = Foo()
foo = complete(foo)
prob = ODEProblem(foo, Dict(foo.x => [1.0, 1]), (0.0, 1.0))

ERROR: MethodError: Cannot `convert` an object of type Symbolics.Arr{Any, 1} to an object of type Equation

[AayushSabharwal]

This is somewhat intentional. Equations aren't scalarized eagerly; this only happens during structural_simplify. Replacing the @named with an @mtkbuild works

[baggepinnen]

In that case, the error message is wrong, and should instead be "structural simplify hasn't been run" or something like that.

The second example still errors though?

[AayushSabharwal]

Broadcasting ~ with symbolics on either side doesn't create an array, it creates an Arr representing the broadcast operation.