handle equality between different system types

Author: baggepinnen

lib/ControlSystemsBase/src/types/TransferFunction.jl

@@ -105,7 +105,7 @@ isrational(::TransferFunction) = true
 #####################################################################
 
 ## EQUALITY ##
-function ==(G1::TransferFunction, G2::TransferFunction)
+function ==(G1::T, G2::T) where T<:TransferFunction
     fields = (:timeevol, :ny, :nu, :matrix)
     for field in fields
         if getproperty(G1, field) != getproperty(G2, field)
@@ -115,6 +115,8 @@ function ==(G1::TransferFunction, G2::TransferFunction)
     return true
 end
 
+==(G1::TransferFunction, G2::TransferFunction) = ==(promote(G1,G2)...)
+
 ## Approximate ##
 function isapprox(G1::TransferFunction, G2::TransferFunction; kwargs...)
     G1, G2 = promote(G1, G2)

lib/ControlSystemsBase/test/test_zpk.jl

@@ -153,7 +153,7 @@ k = 0.3
 @test eltype(fill(zpk(1,0.005)/zpk(2, 0.005),2)) <: TransferFunction
 @test eltype(fill(zpk(1)+1,2)) <: TransferFunction
 
-@test eltype([tf(1,1), zpk(1,1)]) <: TransferFunction
+@test eltype([tf(1,1), zpk(1,1)]) <: LTISystem
 
 zpk(tf([1 2; 3 4])) == zpk([1 2; 3 4])