Merge branch 'master' into check_iv_is_par

src/systems/abstractsystem.jl

@@ -2519,18 +2519,16 @@ function extend(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol = nam
     eqs = union(get_eqs(basesys), get_eqs(sys))
     sts = union(get_unknowns(basesys), get_unknowns(sys))
     ps = union(get_ps(basesys), get_ps(sys))
-    base_deps = parameter_dependencies(basesys)
-    deps = parameter_dependencies(sys)
-    dep_ps = isnothing(base_deps) ? deps :
-             isnothing(deps) ? base_deps : union(base_deps, deps)
+    dep_ps = union_nothing(parameter_dependencies(basesys), parameter_dependencies(sys))
     obs = union(get_observed(basesys), get_observed(sys))
     cevs = union(get_continuous_events(basesys), get_continuous_events(sys))
     devs = union(get_discrete_events(basesys), get_discrete_events(sys))
     defs = merge(get_defaults(basesys), get_defaults(sys)) # prefer `sys`
+    meta = union_nothing(get_metadata(basesys), get_metadata(sys))
     syss = union(get_systems(basesys), get_systems(sys))
     args = length(ivs) == 0 ? (eqs, sts, ps) : (eqs, ivs[1], sts, ps)
     kwargs = (parameter_dependencies = dep_ps, observed = obs, continuous_events = cevs,
-        discrete_events = devs, defaults = defs, systems = syss,
+        discrete_events = devs, defaults = defs, systems = syss, metadata = meta,
         name = name, gui_metadata = gui_metadata)
 
     # collect fields specific to some system types

src/utils.jl

@@ -1,3 +1,14 @@
+"""
+    union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
+
+Unite x and y gracefully when they could be nothing. If neither is nothing, x and y are united normally. If one is nothing, the other is returned unmodified. If both are nothing, nothing is returned.
+"""
+function union_nothing(x::Union{T1, Nothing}, y::Union{T2, Nothing}) where {T1, T2}
+    isnothing(x) && return y # y can be nothing or something
+    isnothing(y) && return x # x can be nothing or something
+    return union(x, y) # both x and y are something and can be united normally
+end
+
 get_iv(D::Differential) = D.x
 
 function make_operation(@nospecialize(op), args)

test/odesystem.jl

@@ -1216,6 +1216,20 @@ end
     @test all(sol2[x] .== 1.0)
 end
 
+# https://github.com/SciML/ModelingToolkit.jl/issues/2502
+@testset "Extend systems with a field that can be nothing" begin
+    A = Dict(:a => 1)
+    B = Dict(:b => 2)
+    @named A1 = ODESystem(Equation[], t, [], [])
+    @named B1 = ODESystem(Equation[], t, [], [])
+    @named A2 = ODESystem(Equation[], t, [], []; metadata = A)
+    @named B2 = ODESystem(Equation[], t, [], []; metadata = B)
+    @test ModelingToolkit.get_metadata(extend(A1, B1)) == nothing
+    @test ModelingToolkit.get_metadata(extend(A1, B2)) == B
+    @test ModelingToolkit.get_metadata(extend(A2, B1)) == A
+    @test Set(ModelingToolkit.get_metadata(extend(A2, B2))) == Set(A ∪ B)
+end
+
 # https://github.com/SciML/ModelingToolkit.jl/issues/2859
 @testset "Initialization with defaults from observed equations (edge case)" begin
     @variables x(t) y(t) z(t)