add fix for extending a Nonlinear system with no independent variables

add fix for extending a Nonlinear system with no independent variables

fix incorrect issue number in comment

Author: stonepreston

src/systems/abstractsystem.jl

@@ -826,7 +826,12 @@ by default.
 function extend(sys::AbstractSystem, basesys::AbstractSystem; name::Symbol=nameof(sys))
     T = SciMLBase.parameterless_type(basesys)
     iv = independent_variable(basesys)
-    sys = convert_system(T, sys, iv)
+    if iv === nothing
+        sys = convert_system(T, sys)
+    else
+        sys = convert_system(T, sys, iv)
+    end
+    
     eqs = union(equations(basesys), equations(sys))
     sts = union(states(basesys), states(sys))
     ps = union(parameters(basesys), parameters(sys))

test/nonlinearsystem.jl

@@ -136,3 +136,24 @@ np = NonlinearProblem(ns, [0,0,0], [1,2,3], jac=true, sparse=true)
        end
        issue819()
 end
+
+# issue #1115
+@testset "Extending a NonlinearSystem with no iv" begin
+    @parameters a b
+    @variables x y
+    eqs1 = [
+        0 ~ a * x
+    ]
+    eqs2 = [
+        0 ~ b * y
+    ]
+
+    @named sys1 = NonlinearSystem(eqs1, [x], [a])
+    @named sys2 = NonlinearSystem(eqs2, [y], [b])
+    @named sys3 = extend(sys1, sys2)
+
+    @test isequal(union(Set(parameters(sys1)), Set(parameters(sys2))), Set(parameters(sys3)))
+    @test isequal(union(Set(states(sys1)), Set(states(sys2))), Set(states(sys3)))
+    @test isequal(union(Set(equations(sys1)), Set(equations(sys2))), Set(equations(sys3)))
+end
+