feat: make deprecated ODESystem alias System type

src/ModelingToolkit.jl

@@ -138,6 +138,9 @@ $(TYPEDEF)
 TODO
 """
 abstract type AbstractSystem end
+# Solely so that `ODESystem` can be deprecated and still act as a valid type.
+# See `deprecations.jl`.
+abstract type IntermediateDeprecationSystem <: AbstractSystem end
 
 function independent_variable end
 
@@ -275,7 +278,7 @@ PrecompileTools.@compile_workload begin
 end
 
 export ODEFunction, convert_system_indepvar,
-       System, OptimizationSystem, JumpSystem, SDESystem, NonlinearSystem
+       System, OptimizationSystem, JumpSystem, SDESystem, NonlinearSystem, ODESystem
 export SDEFunction
 export SystemStructure
 export DiscreteProblem, DiscreteFunction

src/deprecations.jl

@@ -9,7 +9,15 @@ macro mtkbuild(exprs...)
     end |> esc
 end
 
-for T in [:ODESystem, :NonlinearSystem, :DiscreteSystem, :ImplicitDiscreteSystem]
+const ODESystem = IntermediateDeprecationSystem
+
+function IntermediateDeprecationSystem(args...; kwargs...)
+    Base.depwarn("`ODESystem(args...; kwargs...)` is deprecated. Use `System(args...; kwargs...) instead`.", :ODESystem)
+
+    return System(args...; kwargs...)
+end
+
+for T in [:NonlinearSystem, :DiscreteSystem, :ImplicitDiscreteSystem]
     @eval @deprecate $T(args...; kwargs...) System(args...; kwargs...)
 end
 

src/systems/system.jl

@@ -24,7 +24,7 @@ structure.
 
 $(TYPEDFIELDS)
 """
-struct System <: AbstractSystem
+struct System <: IntermediateDeprecationSystem
     """
     $INTERNAL_FIELD_WARNING
     A unique integer tag for the system.

test/odesystem.jl

@@ -1600,3 +1600,17 @@ end
     @test getmetadata(sys, TestMeta, nothing) == "test"
     @test getmetadata(sys2, TestMeta, nothing) == "test"
 end
+
+struct TestWrapper
+    sys::ODESystem
+end
+
+@testset "`ODESystem` is a type" begin
+    @variables x(t)
+    @named sys = ODESystem(D(x) ~ x, t)
+    @test sys isa ODESystem
+    @test sys isa System
+    arr = ODESystem[]
+    @test_nowarn push!(arr, sys)
+    @test_nowarn TestWrapper(sys)
+end