feat: ensure causal connectors generate causally ordered equations

Author: AayushSabharwal

src/systems/connectors.jl

@@ -780,11 +780,11 @@ function generate_connection_equations_and_stream_connections(
                     inner_output = cvert
                 end
             end
-            root, rest = Iterators.peel(cset)
-            root_var = variable_from_vertex(sys, root)
-            for cvert in rest
-                var = variable_from_vertex(sys, cvert)
-                push!(eqs, root_var ~ var)
+            root_vert = something(inner_output, outer_input)
+            root_var = variable_from_vertex(sys, root_vert)
+            for cvert in cset
+                isequal(cvert, root_vert) && continue
+                push!(eqs, variable_from_vertex(sys, cvert) ~ root_var)
             end
         elseif vtype === Stream
             push!(stream_connections, cset)

test/components.jl

@@ -335,3 +335,12 @@ end
     sys = complete(outer)
     @test getmetadata(sys, Int, nothing) == "test"
 end
+
+@testset "Causal connections generate causal equations" begin
+    @named input = RealInput()
+    @named output = RealOutput()
+    @named sys = System([connect(output, input)], t; systems = [input, output])
+    eq = only(equations(expand_connections(sys)))
+    # as opposed to `output.u ~ input.u`
+    @test isequal(eq, input.u ~ output.u)
+end