Added a test for SISO initial value set

Author: Brad Carman

test/Blocks/utils.jl

@@ -1,5 +1,6 @@
 using ModelingToolkitStandardLibrary.Blocks
 using ModelingToolkit
+using OrdinaryDiffEq
 
 @testset "Array Guesses" begin
     for (block, guess) in [
@@ -23,6 +24,22 @@ end
     end
 end
 
+@testset "SISO Check" begin
+    k, w, d = 1.0, 1.0, 0.5
+    @named c = Constant(; k = 1)
+    @named so = SecondOrder(; k = k, w = w, d = d, xd = 1)
+    @named iosys = ODESystem(connect(c.output, so.input), t, systems = [so, c])
+    sys = structural_simplify(iosys)
+
+    initsys = ModelingToolkit.generate_initializesystem(sys)
+    initsys = structural_simplify(initsys)
+    initprob = NonlinearProblem(initsys, [t=>0])
+    initsol = solve(initprob)
+
+    @test initsol[sys.so.xd] == 1.0 
+    @test initsol[sys.so.u] == 1.0
+end
+
 @test_deprecated RealInput(; name = :a, u_start = 1.0)
 @test_deprecated RealInput(; name = :a, nin = 2, u_start = ones(2))
 @test_deprecated RealOutput(; name = :a, u_start = 1.0)