derivative must always return Constant or Expression! fix #545

Author: shashi

src/differentials.jl

@@ -192,9 +192,11 @@ for (modu, fun, arity) ∈ DiffRules.diffrules()
     end
 end
 
-derivative(::typeof(+), args::NTuple{N,Any}, ::Val) where {N} = 1
+derivative(::typeof(+), args::NTuple{N,Any}, ::Val) where {N} = Constant(1)
+derivative(::typeof(-), args::NTuple{2,Any}, ::Val{1}) = Constant(1)
+derivative(::typeof(-), args::NTuple{2,Any}, ::Val{2}) = Constant(-1)
 derivative(::typeof(*), args::NTuple{N,Any}, ::Val{i}) where {N,i} = make_operation(*, deleteat!(collect(args), i))
-derivative(::typeof(one), args::Tuple{<:Any}, ::Val) = 0
+derivative(::typeof(one), args::Tuple{<:Any}, ::Val) = Constant(0)
 
 function count_order(x)
     @assert !(x isa Symbol) "The variable $x must have an order of differentiation that is greater or equal to 1!"

test/derivatives.jl

@@ -50,6 +50,13 @@ test_equal(jac[3,1], y)
 test_equal(jac[3,2], x)
 test_equal(jac[3,3], -1β)
 
+# issue #545
+z = t + t^2
+test_equal(expand_derivatives(D(z)), 1 + t * 2)
+
+z = t-2t
+test_equal(expand_derivatives(D(z)), -1)
+
 # Variable dependence checking in differentiation
 @variables a(t) b(a)
 @test !isequal(D(b), 0)