feat: mark getproperty(::AbstractSystem, ::Symbol) as non-differentiable

Author: AayushSabharwal

ext/MTKChainRulesCoreExt.jl

@@ -103,4 +103,6 @@ function ChainRulesCore.rrule(
     newbuf, pullback
 end
 
+ChainRulesCore.@non_differentiable Base.getproperty(sys::MTK.AbstractSystem, x::Symbol)
+
 end

test/extensions/ad.jl

@@ -124,3 +124,13 @@ fwd, back = ChainRulesCore.rrule(remake_buffer, sys, ps, idxs, vals)
     nsol = solve(nprob, NewtonRaphson())
     @test nsol[1] ≈ 10.0 / 1.0 + 9.81 * 1.0 / 2 # anal free fall solution is y = v0*t - g*t^2/2 -> v0 = y/t + g*t/2
 end
+
+@testset "`sys.var` is non-differentiable" begin
+    @variables x(t)
+    @mtkbuild sys = ODESystem(D(x) ~ x, t)
+    prob = ODEProblem(sys, [x => 1.0], (0.0, 1.0))
+
+    grad = Zygote.gradient(prob) do prob
+        prob[sys.x]
+    end
+end