Merge pull request SciML#2574 from SciML/fixedpoint_sub_diff

Handle derivatives of observed variables

Author: ChrisRackauckas

src/systems/nonlinear/initializesystem.jl

@@ -20,6 +20,8 @@ function generate_initializesystem(sys::ODESystem;
 
     eqs_diff = eqs[idxs_diff]
     diffmap = Dict(getfield.(eqs_diff, :lhs) .=> getfield.(eqs_diff, :rhs))
+    observed_diffmap = Dict(Differential(get_iv(sys)).(getfield.((observed(sys)), :lhs)) .=>
+        Differential(get_iv(sys)).(getfield.((observed(sys)), :rhs)))
 
     full_states = unique([sts; getfield.((observed(sys)), :lhs)])
     set_full_states = Set(full_states)
@@ -36,7 +38,9 @@ function generate_initializesystem(sys::ODESystem;
             filtered_u0 = Pair[]
             for x in u0map
                 y = get(schedule.dummy_sub, x[1], x[1])
+                y = ModelingToolkit.fixpoint_sub(y, observed_diffmap)
                 y = get(diffmap, y, y)
+
                 if y isa Symbolics.Arr
                     _y = collect(y)