fix: handle unknowns inside callable parameters when topsorting equations

Author: AayushSabharwal

src/utils.jl

@@ -383,6 +383,14 @@ function vars!(vars, eq::Equation; op = Differential)
 end
 function vars!(vars, O; op = Differential)
     if isvariable(O)
+        if iscalledparameter(O)
+            f = getcalledparameter(O)
+            push!(vars, f)
+            for arg in arguments(O)
+                vars!(vars, arg; op)
+            end
+            return vars
+        end
         return push!(vars, O)
     end
     if symbolic_type(O) == NotSymbolic() && O isa AbstractArray

test/structural_transformation/utils.jl

@@ -32,3 +32,11 @@ se = collect(StructuralTransformations.edges(graph))
 @test se == mapreduce(vcat, enumerate(graph.fadjlist)) do (s, d)
     StructuralTransformations.BipartiteEdge.(s, d)
 end
+
+@testset "observed2graph handles unknowns inside callable parameters" begin
+    @variables x(t) y(t)
+    @parameters p(..)
+    g, _ = ModelingToolkit.observed2graph([y ~ p(x), x ~ 0], [y, x])
+    @test ModelingToolkit.𝑠neighbors(g, 1) == [2]
+    @test ModelingToolkit.𝑑neighbors(g, 2) == [1]
+end