Diffvar type check introduced in v9.63 causes error with complex diffvars

[david-pl]

The recently introduced type check here

ModelingToolkit.jl/src/utils.jl

Line 1240 in 57c79e9

!(symtype(diffvar) === Real || eltype(symtype(diffvar)) === Real) &&

throws an error for previously working examples / tests in QuantumCumulants.jl.

Tests error with, e.g.

  ArgumentError: Differential variable var"⟨a′*a⟩"(t) has type Complex. Differential variables should not be concretely typed.

The check is too restrictive IMHO, since it's not strictly necessary for things to be <: Real here. Would it be possible to loosen the constraints? Would it be enough to check whether the type is abstract and <: Number?

Also, if you want to be picky: the check doesn't actually do what the error message says. Checking for symtype(x) === Real isn't actually checking for concretely typed variables.

[ChrisRackauckas]

<: Number should be fine.