Merge pull request #4750 from SciML/as/const-arg-of-op

fix: `Const` argument of operator is not considered a variable

Author: AayushSabharwal

lib/ModelingToolkitBase/src/utils.jl

@@ -846,6 +846,7 @@ function collect_vars!(unknowns::OrderedSet{SymbolicT}, parameters::OrderedSet{S
     SU.search_variables!(vars, expr; is_atomic = OperatorIsAtomic{op}())
     for var in vars
         Moshi.Match.@match var begin
+            BSImpl.Const() => nothing
             BSImpl.Term(; f, args) && if f isa op end => begin
                 validate_operator(f, args, iv; context = expr)
                 isempty(args) && continue

lib/ModelingToolkitBase/test/system_building.jl

@@ -1,5 +1,6 @@
 using ModelingToolkitBase
 using ModelingToolkitBase: t_nounits as t, D_nounits as D
+import SymbolicUtils as SU
 
 @testset "`state_priorities` and `irreducibles` kwargs take priority over metadata" begin
     @variables x(t) [state_priority = 3] y(t) [irreducible = false]
@@ -62,3 +63,12 @@ end
     sys = ModelingToolkitBase.set_necessary_initial_conditions(sys, ics)
     @test_throws ModelingToolkitBase.MissingNecessaryInitialConditionsError ODEProblem(sys, [x => 1], (0.0, 1.0))
 end
+
+struct MyOp <: SU.Operator end
+ModelingToolkitBase.validate_operator(::MyOp, args...; kws...) = nothing
+
+@testset "Const argument of operator is not considered a variable" begin
+    @variables x(t)
+    @named sys = System([D(x) ~ x + SU.term(MyOp(), [0.0]; type = Real, shape = SU.ShapeVecT())], t)
+    @test issetequal(unknowns(sys), [x])
+end