feat: support zero-argument operators

Author: AayushSabharwal

src/systems/clock_inference.jl

@@ -285,6 +285,7 @@ function is_time_domain_conversion(v)
     o isa Operator || return false
     itd = input_timedomain(o)
     allequal(itd) || return true
+    isempty(itd) && return true
     otd = output_timedomain(o)
     itd[1] == otd || return true
     return false

src/systems/systemstructure.jl

@@ -456,7 +456,8 @@ function TearingState(sys; quick_cancel = false, check = true, sort_eqs = true)
 
             if !symbolic_contains(v, dvs)
                 isvalid = iscall(v) &&
-                          (operation(v) isa Shift || is_transparent_operator(operation(v)))
+                          (operation(v) isa Shift || isempty(arguments(v)) ||
+                           is_transparent_operator(operation(v)))
                 v′ = v
                 while !isvalid && iscall(v′) && operation(v′) isa Union{Differential, Shift}
                     v′ = arguments(v′)[1]

src/utils.jl

@@ -633,15 +633,21 @@ can be checked using `check_scope_depth`.
 This function should return `nothing`.
 """
 function collect_vars!(unknowns, parameters, expr, iv; depth = 0, op = Symbolics.Operator)
+    expr = unwrap(expr)
     if issym(expr)
         return collect_var!(unknowns, parameters, expr, iv; depth)
     end
-    for var in vars(expr; op)
-        while iscall(var) && operation(var) isa op
-            validate_operator(operation(var), arguments(var), iv; context = expr)
-            var = arguments(var)[1]
+    varsbuf = OrderedSet()
+    vars!(varsbuf, expr; op)
+    for var in varsbuf
+        if iscall(var) && operation(var) isa op
+            args = arguments(var)
+            validate_operator(operation(var), args, iv; context = expr)
+            isempty(args) && continue
+            push!(varsbuf, args[1])
+        else
+            collect_var!(unknowns, parameters, var, iv; depth)
         end
-        collect_var!(unknowns, parameters, var, iv; depth)
     end
     return nothing
 end
@@ -1184,4 +1190,4 @@ function wrap_with_D(n, D, repeats)
     else
         wrap_with_D(D(n), D, repeats - 1)
     end
-end
+end