Fix differentiation of variables

Author: HarrisonGrodin

src/differentials.jl

@@ -21,6 +21,7 @@ function expand_derivatives(O::Operation)
     if O.op isa Differential
         D = O.op
         o = O.args[1]
+        isa(o, Variable) && return O
         return simplify_constants(sum(i->Derivative(o,i)*expand_derivatives(D(o.args[i])),1:length(o.args)))
     end
 

test/derivatives.jl

@@ -40,3 +40,5 @@ jac = ModelingToolkit.calculate_jacobian(sys)
 @Unknown a(t) b(a)
 @test D(b) ≠ Constant(0)
 
+@test expand_derivatives(D(x * y)) == simplify_constants(y*D(x) + x*D(y))
+@test_broken expand_derivatives(D(x * y)) == simplify_constants(D(x)*y + x*D(y))