fix #2215

Author: baggepinnen

src/systems/abstractsystem.jl

@@ -1424,7 +1424,12 @@ function markio!(state, orig_inputs, inputs, outputs; check = true)
     outputset = Dict{Any, Bool}(o => false for o in outputs)
     for (i, v) in enumerate(fullvars)
         if v in keys(inputset)
-            v = setio(v, true, false)
+            if v in keys(outputset)
+                v = setio(v, true, true)
+                outputset[v] = true
+            else
+                v = setio(v, true, false)
+            end
             inputset[v] = true
             fullvars[i] = v
         elseif v in keys(outputset)

test/linearize.jl

@@ -19,6 +19,13 @@ lsys, ssys = linearize(sys, [r], [y])
 @test lsys.C[] == 1
 @test lsys.D[] == 0
 
+lsys, ssys = linearize(sys, [r], [r])
+
+@test lsys.A[] == -2
+@test lsys.B[] == 1
+@test lsys.C[] == 0
+@test lsys.D[] == 1
+
 ##
 ```