[Enhancement] make `Complex{Symbolics.Num}` a `ScalarSymbolic()` similar to `Symbolics.Num`

[OliverAh]

In src/variable.jl:331

SymbolicIndexingInterface.symbolic_type(::Type{Symbolics.Num}) = ScalarSymbolic()

the symbolic_type of a Symbolics.Num is inferred to be a ScalarSymbolic.
However, when working with complex-valued symbols, they are currently inferred to be a NotSymbolic. This causes some issues in substitution of matrices of symbols, where they are inserted as SymbolicUtils.Const instead of SymbolicUtils.array_literal. This in return prevents get_variables to find the symbols in the matrix.

Solution

Would it be possible to add also the following line to the file?

SymbolicIndexingInterface.symbolic_type(::Type{Complex{Symbolics.Num}}) = ScalarSymbolic()

Demonstrator

import Symbolics
import SymbolicUtils

# Uncomment next line to see new behaviour.
# SymbolicUtils.SymbolicIndexingInterface.symbolic_type(::Type{Complex{Symbolics.Num}}) = SymbolicUtils.ScalarSymbolic()

θ, λ = Symbolics.@variables θ::Real λ::Real

real_f =    Symbolics.variable(:real_f;    T=Symbolics.FnType{Tuple{Vararg{Number}}, Number, Nothing})(θ)
complex_f = Symbolics.variable(:complex_f; T=Symbolics.FnType{Tuple{Vararg{Number}}, Number, Nothing})(λ)

real_mat =    Matrix([exp(    θ) 0.0; 0.0 0.0])
complex_mat = Matrix([exp(1im*λ) 0.0; 0.0 0.0])

expr = real_f * complex_f

real_subs =    Symbolics.substitute(expr, Dict(real_f => real_mat))  # substitutes a SymbolicUtils.array_literal
complex_subs = Symbolics.substitute(expr, Dict(complex_f => complex_mat)) # substitutes a SymbolicUtils.Const

real_vars =    Symbolics.get_variables(real_subs)
complex_vars = Symbolics.get_variables(complex_subs) # does not find λ

The added line would make also the matrix in complex_subs to be a array_literal (same as in real_subs), and get_variables would then be able to find λ.

Tested with
SymbolicUtils v4.45.0
Symbolics v7.36.0