fix: partially fix observed generation for discrete-time (sub)systems

Author: AayushSabharwal

src/systems/diffeqs/odesystem.jl

@@ -378,7 +378,7 @@ function build_explicit_observed_function(sys, ts;
         checkbounds = true,
         drop_expr = drop_expr,
         ps = full_parameters(sys),
-        op = Differential,
+        op = Operator,
         throw = true)
     if (isscalar = !(ts isa AbstractVector))
         ts = [ts]

test/clock.jl

@@ -503,3 +503,5 @@ prob = ODEProblem(model, [], (0.0, 10.0))
 sol = solve(prob, Tsit5(), kwargshandle = KeywordArgSilent)
 
 @test sol.prob.kwargs[:disc_saved_values][1].t == sol.t[1:2:end] # Test that the discrete-tiem system executed at every step of the continuous solver. The solver saves each time step twice, one state value before discrete affect and one after.
+@test_nowarn ModelingToolkit.build_explicit_observed_function(
+    model, model.counter.ud(k - 1))(sol.u[1], prob.p..., sol.t[1])