Do not attempt to promote to Union

[baggepinnen]

Sometimes symtype returns a union like Union{Float64, Int64}

julia> @variables x::Float64
1-element Vector{Num}:
 x

julia> ModelingToolkit.symtype(ifelse(x > 0, x, 0))
Union{Float64, Int64}

and then

buf = get!(() -> Any[], cachevars, T)

fails.

This is (very) likely to occur with JSML, which appears to type things with Float64

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

This is (very) likely to occur with JSML, which appears to type things with Float64

It shouldn't since that's just wrong.

[AayushSabharwal]

Yeah ::Float64 annotations might cause problems with putting duals in some places.

[AayushSabharwal]

Also, isn't ifelse(x > 0, x, 0) inherently type-unstable? Regardless of symbolic stuff it would be better to write ifelse(x > 0, x, 0.0) anyway (or zero(x)).

[ChrisRackauckas]

We should enforce the type checks soon. If we do that, then code like this will just error during autodiff, as the user should expect because the assertion would fail.

[baggepinnen]

It is type unstable, but a component definition like this

@component function Ramp(; name, start_time=0, offset=0, duration=nothing, height=nothing)
  params = @parameters begin
    (start_time::Float64 = start_time)
    (offset::Float64 = offset)
    (duration::Float64 = duration)
    (height::Float64 = height)
  end
  vars = @variables begin
    y(t), [output = true]
  end
  eqs = Equation[
    y ~ ifelse(start_time < t, ifelse(t < start_time + duration, offset + (t - start_time) * height / duration, offset + height), offset)
  ]
  return ODESystem(eqs, t, vars, params; systems = [], name)
end

appears to result in this error where the default values for the parameters, 0::Int, somehow appears inside the ifelse expression despite the declaration offset::Float64

[AayushSabharwal]

If a constant appears inside the expression instead of the symbolic expression, that's a much bigger problem with a different solution 😅 since that means the model is incorrectly parsed and changing offset in the ODEProblem won't actually change the ifelse argument.

[baggepinnen]

@AayushSabharwal The WaterTreatmentPlant scirpt I pointed you to should reproduce the error that prompted this PR as well