test: test namespacing of variables of variables

Author: AayushSabharwal

test/namespacing.jl

@@ -1,5 +1,6 @@
 using ModelingToolkit
-using ModelingToolkit: t_nounits as t, D_nounits as D, iscomplete, does_namespacing
+using ModelingToolkit: t_nounits as t, D_nounits as D, iscomplete, does_namespacing,
+                       renamespace
 
 @variables x(t)
 @parameters p
@@ -24,3 +25,23 @@ nsys = toggle_namespacing(sys, false)
 
 @test_throws ["namespacing", "inner"] System(
     Equation[], t; systems = [nsys], name = :a)
+
+@testset "Variables of variables" begin
+    @variables x(t) y(x)
+    @named inner = System([D(x) ~ x, y ~ 2x + 1], t)
+    @test issetequal(unknowns(inner), [x, y])
+    ss = mtkcompile(inner)
+    @test isequal(only(unknowns(ss)), x)
+    @test isequal(only(observed(ss)), y ~ 2x + 1)
+
+    @named sys = System(Equation[], t; systems = [inner])
+    xx, yy = let sys = inner
+        xx = renamespace(sys, x)
+        yy = only(@variables y(xx))
+        xx, renamespace(sys, yy)
+    end
+    @test issetequal(unknowns(sys), [xx, yy])
+    ss = mtkcompile(sys)
+    @test isequal(only(unknowns(ss)), xx)
+    @test isequal(only(observed(ss)), yy ~ 2xx + 1)
+end