Specialize feedback index keywords for strict compilation (#1097)

* Improve constant propagation for compiled control analyses

* Replace forced inlining with index specialization and constant propagation

* Limit the proposal to feedback index specialization

Author: baggepinnen

lib/ControlSystemsBase/src/connections.jl

@@ -351,8 +351,9 @@ The manual section [From block diagrams to code](https://juliacontrol.github.io/
 See Zhou, Doyle, Glover (1996) for similar (somewhat less symmetric) formulas.
 """
 function feedback(sys1::AbstractStateSpace, sys2::AbstractStateSpace;
-    U1=:, Y1=:, U2=:, Y2=:, W1=:, Z1=:, W2=Int[], Z2=Int[],
-    Wperm=:, Zperm=:, pos_feedback::Bool=false)
+    U1::TU1=:, Y1::TY1=:, U2::TU2=:, Y2::TY2=:, W1::TW1=:, Z1::TZ1=:,
+    W2::TW2=Int[], Z2::TZ2=Int[], Wperm::TWperm=:, Zperm::TZperm=:,
+    pos_feedback::Bool=false) where {TU1, TY1, TU2, TY2, TW1, TZ1, TW2, TZ2, TWperm, TZperm}
 
     timeevol = common_timeevol(sys1,sys2)
     T = Base.promote_type(numeric_type(sys1), numeric_type(sys2))
@@ -371,10 +372,10 @@ function feedback(sys1::AbstractStateSpace, sys2::AbstractStateSpace;
     Z2 isa Number && (Z2 = [Z2])
 
     if (U1 isa Colon ? size(sys1, 2) : length(U1)) != (Y2 isa Colon ? size(sys2, 1) : length(Y2))
-        error("Lengths of U1 ($U1) and Y2 ($Y2) must be equal")
+        error("Lengths of U1 and Y2 must be equal")
     end
     if (U2 isa Colon ? size(sys2, 2) : length(U2)) != (Y1 isa Colon ? size(sys1, 1) : length(Y1))
-        error("Lengths of U2 ($U2) and Y1 ($Y1) must be equal")
+        error("Lengths of U2 and Y1 must be equal")
     end
 
     α = pos_feedback ? 1 : -1 # The sign of feedback