Fix #688

Author: YingboMa

src/differentials.jl

@@ -87,7 +87,10 @@ function expand_derivatives(O::Term, simplify=true; occurances=nothing)
             if isequal(o.op.x, D.x)
                 return O
             else
-                return expand_derivatives(o.op(expand_derivatives(D(o.args[1]), false)), simplify)
+                inner = expand_derivatives(D(o.args[1]), false)
+                # if the inner expression is not expandable either, return
+                operation(inner) isa Differential && return O
+                return expand_derivatives(o.op(inner), simplify)
             end
         end
 

test/derivatives.jl

@@ -113,3 +113,12 @@ test_equal(expand_derivatives(Differential(x)(L) - ∂ₜ(Differential(∂ₜ(x)
 
 @variables x2(t)
 @test isequal(expand_derivatives(Differential(x)(2 * x + x2 * x)), 2 + x2)
+
+@parameters x y
+@variables u(..)
+@derivatives Dy'~y
+@derivatives Dx'~x
+dxyu = Dx(Dy(u(x,y)))
+@test isequal(expand_derivatives(dxyu), dxyu)
+dxxu = Dx(Dx(u(x,y)))
+@test isequal(expand_derivatives(dxxu), dxxu)