test: test improved causal variable connection semantics

Author: AayushSabharwal

test/causal_variables_connection.jl

@@ -90,3 +90,34 @@ end
         @test matrices.D[] == 0
     end
 end
+
+@testset "Outside input to inside input connection" begin
+    @mtkmodel Inner begin
+        @variables begin
+            x(t), [input = true]
+            y(t), [output = true]
+        end
+        @equations begin
+            y ~ x
+        end
+    end
+    @mtkmodel Outer begin
+        @variables begin
+            u(t), [input = true]
+            v(t), [output = true]
+        end
+        @components begin
+            inner = Inner()
+        end
+        @equations begin
+            connect(u, inner.x)
+            connect(inner.y, v)
+        end
+    end
+    @named sys = Outer()
+    ss = toggle_namespacing(sys, false)
+    eqs = equations(expand_connections(sys))
+    @test issetequal(eqs, [ss.u ~ ss.inner.x
+                           ss.inner.y ~ ss.inner.x
+                           ss.inner.y ~ ss.v])
+end