fix: improve validation of variables in build_explicit_observed_function

Author: AayushSabharwal

src/systems/diffeqs/odesystem.jl

@@ -476,15 +476,15 @@ function build_explicit_observed_function(sys, ts;
         end
     end
     allsyms = Set(all_symbols(sys))
+    iv = has_iv(sys) ? get_iv(sys) : nothing
     for var in vs
         var = unwrap(var)
         newvar = get(ns_map, var, nothing)
         if newvar !== nothing
             namespace_subs[var] = newvar
             var = newvar
         end
-        if throw && !(var in allsyms) &&
-           (!iscall(var) || operation(var) !== getindex || !(arguments(var)[1] in allsyms))
+        if throw && !var_in_varlist(var, allsyms, iv)
             Base.throw(ArgumentError("Symbol $var is not present in the system."))
         end
     end

src/utils.jl

@@ -1267,3 +1267,22 @@ function symbol_to_symbolic(sys::AbstractSystem, sym; allsyms = all_symbols(sys)
     end
     return sym
 end
+
+"""
+    $(TYPEDSIGNATURES)
+
+Check if `var` is present in `varlist`. `iv` is the independent variable of the system,
+and should be `nothing` if not applicable.
+"""
+function var_in_varlist(var, varlist::AbstractSet, iv)
+    var = unwrap(var)
+    # simple case
+    return var in varlist ||
+           # indexed array symbolic, unscalarized array present
+           (iscall(var) && operation(var) === getindex && arguments(var)[1] in varlist) ||
+           # unscalarized sized array symbolic, all scalarized elements present
+           (symbolic_type(var) == ArraySymbolic() && is_sized_array_symbolic(var) &&
+            all(x -> x in varlist, collect(var))) ||
+           # delayed variables
+           (isdelay(var, iv) && var_in_varlist(operation(var)(iv), varlist, iv))
+end