fix change of var for sys with no equations

src/systems/diffeqs/basic_transformations.jl

@@ -155,7 +155,7 @@ function change_independent_variable(
 
     # Create a utility that performs the chain rule on an expression, followed by insertion of the new independent variable:
     # e.g. (d/dt)(f(t)) -> (d/dt)(f(u(t))) -> df(u(t))/du(t) * du(t)/dt -> df(u)/du * uˍt(u)
-    function transform(ex)
+    function transform(ex::T) where {T}
         # 1) Replace the argument of every function; e.g. f(t) -> f(u(t))
         for var in vars(ex; op = Nothing) # loop over all variables in expression (op = Nothing prevents interpreting "D(f(t))" as one big variable)
             is_function_of_iv1 = iscall(var) && isequal(only(arguments(var)), iv1) # of the form f(t)?
@@ -175,7 +175,7 @@ function change_independent_variable(
         # 3) Set new independent variable
         ex = substitute(ex, iv2_of_iv1 => iv2; fold) # set e.g. u(t) -> u everywhere
         ex = substitute(ex, iv1 => iv1_of_iv2; fold) # set e.g. t -> t(u) everywhere
-        return ex
+        return ex::T
     end
 
     # Use the utility function to transform everything in the system!

test/basic_transformations.jl

@@ -231,3 +231,31 @@ end
     # compare to analytical solution (x(t) = v*t, y(t) = v*t - g*t^2/2)
     @test all(isapprox.(sol[Mx.y], sol[Mx.x - g * (Mx.t_units)^2 / 2]; atol = 1e-10))
 end
+
+@testset "Change independent variable, no equations" begin
+    # make this "look" like the standard library RealInput
+    @mtkmodel Input begin
+        @variables begin
+            u(t)
+        end
+    end
+    @named input_sys = Input()
+    input_sys = complete(input_sys)
+    # test no failures
+    @test change_independent_variable(input_sys, input_sys.u) isa ODESystem
+
+    @mtkmodel NestedInput begin
+        @components begin
+            in = Input()
+        end
+        @variables begin
+            x(t)
+        end
+        @equations begin
+            D(x) ~ in.u
+        end
+    end
+    @named nested_input_sys = NestedInput()
+    nested_input_sys = complete(nested_input_sys; flatten = false)
+    @test change_independent_variable(nested_input_sys, nested_input_sys.x) isa ODESystem
+end