fix promotion of discrete time evolution (#131)

Author: baggepinnen

src/named_systems2.jl

@@ -105,7 +105,7 @@ function Base.convert(::Type{NamedStateSpace{T, S}}, s::U) where {T, S <: Abstra
 end
 
 function Base.convert(::Type{NamedStateSpace{T, S}}, M::AbstractMatrix) where {T, S <: AbstractStateSpace}
-    te = T <: Discrete ? Discrete(ControlSystemsBase.UNDEF_SAMPLEPETIME) : Continuous()
+    te = T <: Discrete ? T(Discrete(ControlSystemsBase.UNDEF_SAMPLEPETIME)) : Continuous()
     NT = numeric_type(S)
     named_ss(ss(NT.(M), te), x = gensym("x"), u = gensym("u"), y = gensym("y"))
 end
@@ -121,7 +121,7 @@ function Base.convert(::Type{NamedStateSpace{T, S}}, s::U) where {T, S <: Abstra
 end
 
 function Base.convert(::Type{NamedStateSpace{T, S}}, N::Number) where {T, S <: AbstractStateSpace}
-    te = T <: Discrete ? Discrete(ControlSystemsBase.UNDEF_SAMPLEPETIME) : Continuous()
+    te = T <: Discrete ? T(Discrete(ControlSystemsBase.UNDEF_SAMPLEPETIME)) : Continuous()
     NT = numeric_type(S)
     named_ss(tf(NT(N), te), x = gensym("x"), u = gensym("u"), y = gensym("y"))
 end

test/test_named_systems2.jl

@@ -499,4 +499,9 @@ isys = inv(s1)
 
 isys = 2/s1
 @test isys.sys == 2/s1.sys
-@test isys.x != s1.x
+@test isys.x != s1.x
+
+# Test https://github.com/JuliaControl/RobustAndOptimalControl.jl/issues/130
+P = named_ss(ssrand(1,1,2, Ts=1.0))
+C = named_ss(ssrand(1,1,2, Ts=1.0))
+@test gangoffour(P, C) isa NTuple{4, NamedStateSpace{Discrete{Float64}, StateSpace{Discrete{Float64}, Float64}}}