fix: fix observed function generation for systems with inputs

Author: AayushSabharwal

src/systems/diffeqs/odesystem.jl

@@ -473,14 +473,18 @@ function build_explicit_observed_function(sys, ts;
         ps = DestructuredArgs.(ps, inbounds = !checkbounds)
     elseif has_index_cache(sys) && get_index_cache(sys) !== nothing
         ps = DestructuredArgs.(reorder_parameters(get_index_cache(sys), ps))
+        if isempty(ps) && inputs !== nothing
+            ps = (:EMPTY,)
+        end
     else
         ps = (DestructuredArgs(ps, inbounds = !checkbounds),)
     end
     dvs = DestructuredArgs(unknowns(sys), inbounds = !checkbounds)
     if inputs === nothing
         args = [dvs, ps..., ivs...]
     else
-        ipts = DestructuredArgs(unwrap.(inputs), inbounds = !checkbounds)
+        inputs = unwrap.(inputs)
+        ipts = DestructuredArgs(inputs, inbounds = !checkbounds)
         args = [dvs, ipts, ps..., ivs...]
     end
     pre = get_postprocess_fbody(sys)

test/input_output_handling.jl

@@ -378,3 +378,16 @@ matrices, ssys = linearize(augmented_sys,
 # P = ss(A,B,C,0)
 # G = ss(matrices...)
 # @test sminreal(G[1, 3]) ≈ sminreal(P[1,1])*dist
+
+@testset "Observed functions with inputs" begin
+    @variables x(t)=0 u(t)=0 [input = true]
+    eqs = [
+        D(x) ~ -x + u
+    ]
+
+    @named sys = ODESystem(eqs, t)
+    (; io_sys,) = ModelingToolkit.generate_control_function(sys, simplify = true)
+    obsfn = ModelingToolkit.build_explicit_observed_function(
+        io_sys, [x + u * t]; inputs = [u])
+    @test obsfn([1.0], [2.0], nothing, 3.0) == [7.0]
+end