Fix independent variable extraction

Author: HarrisonGrodin

src/equations.jl

@@ -33,9 +33,14 @@ function vars!(vars, op)
     args = isa(op, Equation) ? Expression[op.lhs, op.rhs] : op.args
 
     for arg ∈ args
-        isa(arg, Operation) ? vars!(vars, arg) :
-        isa(arg, Variable)  ? push!(vars, arg) :
-        nothing
+        if isa(arg, Operation)
+            vars!(vars, arg)
+        elseif isa(arg, Variable)
+            push!(vars, arg)
+            for dep ∈ arg.dependents
+                push!(vars, dep)
+            end
+        end
     end
 
     return vars

test/system_construction.jl

@@ -91,8 +91,8 @@ eqs = [_x ~ y/C,
        D(x) ~ -A*x,
        D(y) ~ A*x - B*_x]
 de = DiffEqSystem(eqs,[t],[x,y],Variable[_x],[A,B,C])
-de2 = DiffEqSystem(eqs,[t])
-test_vars_extraction(de, de2)
+test_vars_extraction(de, DiffEqSystem(eqs,[t]))
+test_vars_extraction(de, DiffEqSystem(eqs))
 @test eval(ModelingToolkit.generate_ode_function(de))([0.0,0.0],[1.0,2.0],[1,2,3],0.0) ≈ -1/3
 
 # Now nonlinear system with only variables