Fix Constant differentiation

Author: HarrisonGrodin

src/differentials.jl

@@ -13,6 +13,7 @@ function (D::Differential)(x::Variable)
     has_dependent(x, D.x) || return Constant(0)
     return Operation(D, Expression[x])
 end
+(::Differential)(::Constant) = Constant(0)
 Base.:(==)(D1::Differential, D2::Differential) = D1.order == D2.order && D1.x == D2.x
 
 function expand_derivatives(O::Operation)
@@ -21,7 +22,7 @@ function expand_derivatives(O::Operation)
     if O.op isa Differential
         D = O.op
         o = O.args[1]
-        isa(o, Variable) && return O
+        isa(o, Operation) || return O
         return simplify_constants(sum(i->Derivative(o,i)*expand_derivatives(D(o.args[i])),1:length(o.args)))
     end
 

test/derivatives.jl

@@ -42,3 +42,6 @@ jac = ModelingToolkit.calculate_jacobian(sys)
 
 @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))
+
+@test expand_derivatives(D(2t)) == 2
+@test expand_derivatives(D(2x)) == 2D(x)