generate_control_function test and fix

src/inputoutput.jl

@@ -218,6 +218,7 @@ function generate_control_function(sys::AbstractODESystem, inputs = unbound_inpu
     inputs = map(x -> time_varying_as_func(value(x), sys), inputs)
 
     eqs = [eq for eq in full_equations(sys)]
+    eqs = map(subs_constants, eqs)
     if disturbance_inputs !== nothing
         # Set all disturbance *inputs* to zero (we just want to keep the disturbance state)
         subs = Dict(disturbance_inputs .=> 0)

test/input_output_handling.jl

@@ -171,6 +171,16 @@ x = [rand()]
 u = [rand()]
 @test f[1](x, u, p, 1) == -x + u
 
+@testset "Constants substitution" begin
+    @constants c = 2.0
+    @variables x(t)
+    eqs = [D(x) ~ c * x]
+    @named sys = ODESystem(eqs, t, [x], [])
+
+    f, dvs, ps = ModelingToolkit.generate_control_function(sys, simplify = true)
+    @test f[1]([0.5], nothing, nothing, 0.0) == [1.0]
+end
+
 # more complicated system
 
 @variables u(t) [input = true]